<?php

// vim: foldmethod=marker

/* Generic exception class
 */
if (!class_exists('OAuthException')) {

    class OAuthException extends Exception {
        // pass
    }

}
if (!class_exists('OAuthConsumer')) {

    class OAuthConsumer {

        public $key;
        public $secret;

        function __construct($key, $secret, $callback_url = NULL) {
            $this->key = $key;
            $this->secret = $secret;
            $this->callback_url = $callback_url;
        }

        function __toString() {
            return "OAuthConsumer[key=$this->key,secret=$this->secret]";
        }

    }

}
if (!class_exists('OAuthToken')) {

    class OAuthToken {

        // access tokens and request tokens
        public $key;
        public $secret;

        /**
         * key = the token
         * secret = the token secret
         */
        function __construct($key, $secret) {
            $this->key = $key;
            $this->secret = $secret;
        }

        /**
         * generates the basic string serialization of a token that a server
         * would respond to request_token and access_token calls with
         */
        function to_string() {
            return "oauth_token=" .
                    OAuthUtil::urlencode_rfc3986($this->key) .
                    "&oauth_token_secret=" .
                    OAuthUtil::urlencode_rfc3986($this->secret);
        }

        function __toString() {
            return $this->to_string();
        }

    }

}
/**
 * A class for implementing a Signature Method
 * See section 9 ("Signing Requests") in the spec
 */
if (!class_exists('OAuthSignatureMethod')) {

    abstract class OAuthSignatureMethod {

        /**
         * Needs to return the name of the Signature Method (ie HMAC-SHA1)
         * @return string
         */
        abstract public function get_name();

        /**
         * Build up the signature
         * NOTE: The output of this function MUST NOT be urlencoded.
         * the encoding is handled in OAuthRequest when the final
         * request is serialized
         * @param OAuthRequest $request
         * @param OAuthConsumer $consumer
         * @param OAuthToken $token
         * @return string
         */
        abstract public function build_signature($request, $consumer, $token);

        /**
         * Verifies that a given signature is correct
         * @param OAuthRequest $request
         * @param OAuthConsumer $consumer
         * @param OAuthToken $token
         * @param string $signature
         * @return bool
         */
        public function check_signature($request, $consumer, $token, $signature) {
            $built = $this->build_signature($request, $consumer, $token);
            return $built == $signature;
        }

    }

}
/**
 * The HMAC-SHA1 signature method uses the HMAC-SHA1 signature algorithm as defined in [RFC2104] 
 * where the Signature Base String is the text and the key is the concatenated values (each first 
 * encoded per Parameter Encoding) of the Consumer Secret and Token Secret, separated by an '&' 
 * character (ASCII code 38) even if empty.
 *   - Chapter 9.2 ("HMAC-SHA1")
 */
if (!class_exists('OAuthSignatureMethod_HMAC_SHA1')) {

    class OAuthSignatureMethod_HMAC_SHA1 extends OAuthSignatureMethod {

        function get_name() {
            return "HMAC-SHA1";
        }

        public function build_signature($request, $consumer, $token) {
            $base_string = $request->get_signature_base_string();
            $request->base_string = $base_string;
            $key_parts = array(
                $consumer->secret,
                ($token) ? $token->secret : ""
            );
            $key_parts = OAuthUtil::urlencode_rfc3986($key_parts);
            $key = implode('&', $key_parts);
            return base64_encode(hash_hmac('sha1', $base_string, $key, true));
        }

    }

}
/**
 * The PLAINTEXT method does not provide any security protection and SHOULD only be used 
 * over a secure channel such as HTTPS. It does not use the Signature Base String.
 *   - Chapter 9.4 ("PLAINTEXT")
 */
if (!class_exists('OAuthSignatureMethod_PLAINTEXT')) {

    class OAuthSignatureMethod_PLAINTEXT extends OAuthSignatureMethod {

        public function get_name() {
            return "PLAINTEXT";
        }

        /**
         * oauth_signature is set to the concatenated encoded values of the Consumer Secret and 
         * Token Secret, separated by a '&' character (ASCII code 38), even if either secret is 
         * empty. The result MUST be encoded again.
         *   - Chapter 9.4.1 ("Generating Signatures")
         * Please note that the second encoding MUST NOT happen in the SignatureMethod, as
         * OAuthRequest handles this!
         */
        public function build_signature($request, $consumer, $token) {
            $key_parts = array(
                $consumer->secret,
                ($token) ? $token->secret : ""
            );
            $key_parts = OAuthUtil::urlencode_rfc3986($key_parts);
            $key = implode('&', $key_parts);
            $request->base_string = $key;
            return $key;
        }

    }

}
/**
 * The RSA-SHA1 signature method uses the RSASSA-PKCS1-v1_5 signature algorithm as defined in 
 * [RFC3447] section 8.2 (more simply known as PKCS#1), using SHA-1 as the hash function for 
 * EMSA-PKCS1-v1_5. It is assumed that the Consumer has provided its RSA public key in a 
 * verified way to the Service Provider, in a manner which is beyond the scope of this 
 * specification.
 *   - Chapter 9.3 ("RSA-SHA1")
 */
if (!class_exists('OAuthSignatureMethod_RSA_SHA1')) {

    abstract class OAuthSignatureMethod_RSA_SHA1 extends OAuthSignatureMethod {

        public function get_name() {
            return "RSA-SHA1";
        }

        // Up to the SP to implement this lookup of keys. Possible ideas are:
        // (1) do a lookup in a table of trusted certs keyed off of consumer
        // (2) fetch via http using a url provided by the requester
        // (3) some sort of specific discovery code based on request
        //
  // Either way should return a string representation of the certificate
        protected abstract function fetch_public_cert(&$request);

        // Up to the SP to implement this lookup of keys. Possible ideas are:
        // (1) do a lookup in a table of trusted certs keyed off of consumer
        //
  // Either way should return a string representation of the certificate
        protected abstract function fetch_private_cert(&$request);

        public function build_signature($request, $consumer, $token) {
            $base_string = $request->get_signature_base_string();
            $request->base_string = $base_string;
            // Fetch the private key cert based on the request
            $cert = $this->fetch_private_cert($request);
            // Pull the private key ID from the certificate
            $privatekeyid = openssl_get_privatekey($cert);
            // Sign using the key
            $ok = openssl_sign($base_string, $signature, $privatekeyid);
            // Release the key resource
            openssl_free_key($privatekeyid);
            return base64_encode($signature);
        }

        public function check_signature($request, $consumer, $token, $signature) {
            $decoded_sig = base64_decode($signature);
            $base_string = $request->get_signature_base_string();
            // Fetch the public key cert based on the request
            $cert = $this->fetch_public_cert($request);
            // Pull the public key ID from the certificate
            $publickeyid = openssl_get_publickey($cert);
            // Check the computed signature against the one passed in the query
            $ok = openssl_verify($base_string, $decoded_sig, $publickeyid);
            // Release the key resource
            openssl_free_key($publickeyid);
            return $ok == 1;
        }

    }

}
if (!class_exists('OAuthRequest')) {

    class OAuthRequest {

        private $parameters;
        private $http_method;
        private $http_url;
        // for debug purposes
        public $base_string;
        public static $version = '1.0';
        public static $POST_INPUT = 'php://input';

        function __construct($http_method, $http_url, $parameters = NULL) {
            @$parameters or $parameters = array();
            $parameters = array_merge(OAuthUtil::parse_parameters(parse_url($http_url, PHP_URL_QUERY)), $parameters);
            $this->parameters = $parameters;
            $this->http_method = $http_method;
            $this->http_url = $http_url;
        }

        /**
         * attempt to build up a request from what was passed to the server
         */
        public static function from_request($http_method = NULL, $http_url = NULL, $parameters = NULL) {
            $scheme = (!isset($_SERVER['HTTPS']) || $_SERVER['HTTPS'] != "on") ? 'http' : 'https';
            @$http_url or $http_url = $scheme .
                    '://' . $_SERVER['HTTP_HOST'] .
                    ':' .
                    $_SERVER['SERVER_PORT'] .
                    $_SERVER['REQUEST_URI'];
            @$http_method or $http_method = $_SERVER['REQUEST_METHOD'];

            // We weren't handed any parameters, so let's find the ones relevant to
            // this request.
            // If you run XML-RPC or similar you should use this to provide your own
            // parsed parameter-list
            if (!$parameters) {
                // Find request headers
                $request_headers = OAuthUtil::get_headers();

                // Parse the query-string to find GET parameters
                $parameters = OAuthUtil::parse_parameters($_SERVER['QUERY_STRING']);
                // It's a POST request of the proper content-type, so parse POST
                // parameters and add those overriding any duplicates from GET
                if ($http_method == "POST" && @strstr($request_headers["Content-Type"], "application/x-www-form-urlencoded")
                ) {
                    $post_data = OAuthUtil::parse_parameters(
                                    file_get_contents(self::$POST_INPUT)
                    );
                    $parameters = array_merge($parameters, $post_data);
                }
                // We have a Authorization-header with OAuth data. Parse the header
                // and add those overriding any duplicates from GET or POST
                if (@substr($request_headers['Authorization'], 0, 6) == "OAuth ") {
                    $header_parameters = OAuthUtil::split_header(
                                    $request_headers['Authorization']
                    );
                    $parameters = array_merge($parameters, $header_parameters);
                }
            }
            return new OAuthRequest($http_method, $http_url, $parameters);
        }

        /**
         * pretty much a helper function to set up the request
         */
        public static function from_consumer_and_token($consumer, $token, $http_method, $http_url, $parameters = NULL) {
            @$parameters or $parameters = array();
            $defaults = array("oauth_version" => OAuthRequest::$version,
                "oauth_nonce" => OAuthRequest::generate_nonce(),
                "oauth_timestamp" => OAuthRequest::generate_timestamp(),
                "oauth_consumer_key" => $consumer->key);
            if ($token)
                $defaults['oauth_token'] = $token->key;
            $parameters = array_merge($defaults, $parameters);
            return new OAuthRequest($http_method, $http_url, $parameters);
        }

        public function set_parameter($name, $value, $allow_duplicates = true) {
            if ($allow_duplicates && isset($this->parameters[$name])) {
                // We have already added parameter(s) with this name, so add to the list
                if (is_scalar($this->parameters[$name])) {
                    // This is the first duplicate, so transform scalar (string)
                    // into an array so we can add the duplicates
                    $this->parameters[$name] = array($this->parameters[$name]);
                }
                $this->parameters[$name][] = $value;
            } else {
                $this->parameters[$name] = $value;
            }
        }

        public function get_parameter($name) {
            return isset($this->parameters[$name]) ? $this->parameters[$name] : null;
        }

        public function get_parameters() {
            return $this->parameters;
        }

        public function unset_parameter($name) {
            unset($this->parameters[$name]);
        }

        /**
         * The request parameters, sorted and concatenated into a normalized string.
         * @return string
         */
        public function get_signable_parameters() {
            // Grab all parameters
            $params = $this->parameters;
            // Remove oauth_signature if present
            // Ref: Spec: 9.1.1 ("The oauth_signature parameter MUST be excluded.")
            if (isset($params['oauth_signature'])) {
                unset($params['oauth_signature']);
            }
            return OAuthUtil::build_http_query($params);
        }

        /**
         * Returns the base string of this request
         * The base string defined as the method, the url
         * and the parameters (normalized), each urlencoded
         * and the concated with &.
         */
        public function get_signature_base_string() {
            $parts = array(
                $this->get_normalized_http_method(),
                $this->get_normalized_http_url(),
                $this->get_signable_parameters()
            );
            $parts = OAuthUtil::urlencode_rfc3986($parts);
            return implode('&', $parts);
        }

        /**
         * just uppercases the http method
         */
        public function get_normalized_http_method() {
            return strtoupper($this->http_method);
        }

        /**
         * parses the url and rebuilds it to be
         * scheme://host/path
         */
        public function get_normalized_http_url() {
            $parts = parse_url($this->http_url);
            $parts = parse_url($this->http_url) + array('port' => NULL, 'path' => NULL);
            $port = @$parts['port'];
            $scheme = $parts['scheme'];
            $host = $parts['host'];
            $path = @$parts['path'];
            $port or $port = ($scheme == 'https') ? '443' : '80';
            if (($scheme == 'https' && $port != '443') || ($scheme == 'http' && $port != '80')) {
                $host = "$host:$port";
            }
            return "$scheme://$host$path";
        }

        /**
         * builds a url usable for a GET request
         */
        public function to_url() {
            $post_data = $this->to_postdata();
            $out = $this->get_normalized_http_url();
            if ($post_data) {
                $out .= '?' . $post_data;
            }
            return $out;
        }

        /**
         * builds the data one would send in a POST request
         */
        public function to_postdata() {
            return OAuthUtil::build_http_query($this->parameters);
        }

        /**
         * builds the Authorization: header
         */
        public function to_header($realm = null) {
            $first = true;
            if ($realm) {
                $out = 'Authorization: OAuth realm="' . OAuthUtil::urlencode_rfc3986($realm) . '"';
                $first = false;
            } else
                $out = 'Authorization: OAuth';
            $total = array();
            foreach ($this->parameters as $k => $v) {
                if (substr($k, 0, 5) != "oauth")
                    continue;
                if (is_array($v)) {
                    throw new OAuthException('Arrays not supported in headers');
                }
                $out .= ($first) ? ' ' : ',';
                $out .= OAuthUtil::urlencode_rfc3986($k) .
                        '="' .
                        OAuthUtil::urlencode_rfc3986($v) .
                        '"';
                $first = false;
            }
            return $out;
        }

        public function __toString() {
            return $this->to_url();
        }

        public function sign_request($signature_method, $consumer, $token) {
            $this->set_parameter(
                    "oauth_signature_method", $signature_method->get_name(), false
            );
            $signature = $this->build_signature($signature_method, $consumer, $token);
            $this->set_parameter("oauth_signature", $signature, false);
        }

        public function build_signature($signature_method, $consumer, $token) {
            $signature = $signature_method->build_signature($this, $consumer, $token);
            return $signature;
        }

        /**
         * util function: current timestamp
         */
        private static function generate_timestamp() {
            return time();
        }

        /**
         * util function: current nonce
         */
        private static function generate_nonce() {
            $mt = microtime();
            $rand = mt_rand();
            return md5($mt . $rand); // md5s look nicer than numbers
        }

    }

}

if (!class_exists('OAuthServer')) {

    class OAuthServer {

        protected $timestamp_threshold = 300; // in seconds, five minutes
        protected $version = '1.0';             // hi blaine
        protected $signature_methods = array();
        protected $data_store;

        function __construct($data_store) {
            $this->data_store = $data_store;
        }

        public function add_signature_method($signature_method) {
            $this->signature_methods[$signature_method->get_name()] = $signature_method;
        }

        // high level functions

        /**
         * process a request_token request
         * returns the request token on success
         */
        public function fetch_request_token(&$request) {
            $this->get_version($request);
            $consumer = $this->get_consumer($request);
            // no token required for the initial token request
            $token = NULL;
            $this->check_signature($request, $consumer, $token);
            // Rev A change
            $callback = $request->get_parameter('oauth_callback');
            $new_token = $this->data_store->new_request_token($consumer, $callback);
            return $new_token;
        }

        /**
         * process an access_token request
         * returns the access token on success
         */
        public function fetch_access_token(&$request) {
            $this->get_version($request);
            $consumer = $this->get_consumer($request);
            // requires authorized request token
            $token = $this->get_token($request, $consumer, "request");
            $this->check_signature($request, $consumer, $token);
            // Rev A change
            $verifier = $request->get_parameter('oauth_verifier');
            $new_token = $this->data_store->new_access_token($token, $consumer, $verifier);
            return $new_token;
        }

        /**
         * verify an api call, checks all the parameters
         */
        public function verify_request(&$request) {
            $this->get_version($request);
            $consumer = $this->get_consumer($request);
            $token = $this->get_token($request, $consumer, "access");
            $this->check_signature($request, $consumer, $token);
            return array($consumer, $token);
        }

        // Internals from here
        /**
         * version 1
         */
        private function get_version(&$request) {
            $version = $request->get_parameter("oauth_version");
            if (!$version) {
                // Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. 
                // Chapter 7.0 ("Accessing Protected Ressources")
                $version = '1.0';
            }
            if ($version !== $this->version) {
                throw new OAuthException("OAuth version '$version' not supported");
            }
            return $version;
        }

        /**
         * figure out the signature with some defaults
         */
        private function get_signature_method(&$request) {
            $signature_method = @$request->get_parameter("oauth_signature_method");
            if (!$signature_method) {
                // According to chapter 7 ("Accessing Protected Ressources") the signature-method
                // parameter is required, and we can't just fallback to PLAINTEXT
                throw new OAuthException('No signature method parameter. This parameter is required');
            }
            if (!in_array($signature_method, array_keys($this->signature_methods))) {
                throw new OAuthException(
                "Signature method '$signature_method' not supported " .
                "try one of the following: " .
                implode(", ", array_keys($this->signature_methods))
                );
            }
            return $this->signature_methods[$signature_method];
        }

        /**
         * try to find the consumer for the provided request's consumer key
         */
        private function get_consumer(&$request) {
            $consumer_key = @$request->get_parameter("oauth_consumer_key");
            if (!$consumer_key) {
                throw new OAuthException("Invalid consumer key");
            }
            $consumer = $this->data_store->lookup_consumer($consumer_key);
            if (!$consumer) {
                throw new OAuthException("Invalid consumer");
            }
            return $consumer;
        }

        /**
         * try to find the token for the provided request's token key
         */
        private function get_token(&$request, $consumer, $token_type = "access") {
            $token_field = @$request->get_parameter('oauth_token');
            $token = $this->data_store->lookup_token(
                    $consumer, $token_type, $token_field
            );
            if (!$token) {
                throw new OAuthException("Invalid $token_type token: $token_field");
            }
            return $token;
        }

        /**
         * all-in-one function to check the signature on a request
         * should guess the signature method appropriately
         */
        private function check_signature(&$request, $consumer, $token) {
            // this should probably be in a different method
            $timestamp = @$request->get_parameter('oauth_timestamp');
            $nonce = @$request->get_parameter('oauth_nonce');
            $this->check_timestamp($timestamp);
            $this->check_nonce($consumer, $token, $nonce, $timestamp);
            $signature_method = $this->get_signature_method($request);
            $signature = $request->get_parameter('oauth_signature');
            $valid_sig = $signature_method->check_signature(
                    $request, $consumer, $token, $signature
            );

            if (!$valid_sig) {
                throw new OAuthException("Invalid signature");
            }
        }

        /**
         * check that the timestamp is new enough
         */
        private function check_timestamp($timestamp) {
            if (!$timestamp)
                throw new OAuthException(
                'Missing timestamp parameter. The parameter is required'
                );

            // verify that timestamp is recentish
            $now = time();
            if (abs($now - $timestamp) > $this->timestamp_threshold) {
                throw new OAuthException(
                "Expired timestamp, yours $timestamp, ours $now"
                );
            }
        }

        /**
         * check that the nonce is not repeated
         */
        private function check_nonce($consumer, $token, $nonce, $timestamp) {
            if (!$nonce)
                throw new OAuthException(
                'Missing nonce parameter. The parameter is required'
                );

            // verify that the nonce is uniqueish
            $found = $this->data_store->lookup_nonce(
                    $consumer, $token, $nonce, $timestamp
            );
            if ($found) {
                throw new OAuthException("Nonce already used: $nonce");
            }
        }

    }

}

if (!class_exists('OAuthDataStore')) {

    class OAuthDataStore {

        function lookup_consumer($consumer_key) {
            // implement me
        }

        function lookup_token($consumer, $token_type, $token) {
            // implement me
        }

        function lookup_nonce($consumer, $token, $nonce, $timestamp) {
            // implement me
        }

        function new_request_token($consumer, $callback = null) {
            // return a new token attached to this consumer
        }

        function new_access_token($token, $consumer, $verifier = null) {
            // return a new access token attached to this consumer
            // for the user associated with this token if the request token
            // is authorized
            // should also invalidate the request token
        }

    }

}
if (!class_exists('OAuthUtil')) {

    class OAuthUtil {

        public static function urlencode_rfc3986($input) {
            if (is_array($input)) {
                return array_map(array('OAuthUtil', 'urlencode_rfc3986'), $input);
            } else if (is_scalar($input)) {
                return str_replace(
                        '+', ' ', str_replace('%7E', '~', rawurlencode($input))
                );
            } else {
                return '';
            }
        }

        // This decode function isn't taking into consideration the above
        // modifications to the encoding process. However, this method doesn't
        // seem to be used anywhere so leaving it as is.
        public static function urldecode_rfc3986($string) {
            return urldecode($string);
        }

        // Utility function for turning the Authorization: header into
        // parameters, has to do some unescaping
        // Can filter out any non-oauth parameters if needed (default behaviour)
        public static function split_header($header, $only_allow_oauth_parameters = true) {
            $pattern = '/(([-_a-z]*)=("([^"]*)"|([^,]*)),?)/';
            $offset = 0;
            $params = array();
            while (preg_match($pattern, $header, $matches, PREG_OFFSET_CAPTURE, $offset) > 0) {
                $match = $matches[0];
                $header_name = $matches[2][0];
                $header_content = (isset($matches[5])) ? $matches[5][0] : $matches[4][0];
                if (preg_match('/^oauth_/', $header_name) || !$only_allow_oauth_parameters) {
                    $params[$header_name] = OAuthUtil::urldecode_rfc3986($header_content);
                }
                $offset = $match[1] + strlen($match[0]);
            }
            if (isset($params['realm'])) {
                unset($params['realm']);
            }
            return $params;
        }

        // helper to try to sort out headers for people who aren't running apache
        public static function get_headers() {
            if (function_exists('apache_request_headers')) {
                // we need this to get the actual Authorization: header
                // because apache tends to tell us it doesn't exist
                $headers = apache_request_headers();
                // sanitize the output of apache_request_headers because
                // we always want the keys to be Cased-Like-This and arh()
                // returns the headers in the same case as they are in the
                // request
                $out = array();
                foreach ($headers AS $key => $value) {
                    $key = str_replace(
                            " ", "-", ucwords(strtolower(str_replace("-", " ", $key)))
                    );
                    $out[$key] = $value;
                }
            } else {
                // otherwise we don't have apache and are just going to have to hope
                // that $_SERVER actually contains what we need
                $out = array();
                if (isset($_SERVER['CONTENT_TYPE']))
                    $out['Content-Type'] = $_SERVER['CONTENT_TYPE'];
                if (isset($_ENV['CONTENT_TYPE']))
                    $out['Content-Type'] = $_ENV['CONTENT_TYPE'];

                foreach ($_SERVER as $key => $value) {
                    if (substr($key, 0, 5) == "HTTP_") {
                        // this is chaos, basically it is just there to capitalize the first
                        // letter of every word that is not an initial HTTP and strip HTTP
                        // code from przemek
                        $key = str_replace(
                                " ", "-", ucwords(strtolower(str_replace("_", " ", substr($key, 5))))
                        );
                        $out[$key] = $value;
                    }
                }
            }
            return $out;
        }

        // This function takes a input like a=b&a=c&d=e and returns the parsed
        // parameters like this
        // array('a' => array('b','c'), 'd' => 'e')
        public static function parse_parameters($input) {
            if (!isset($input) || !$input)
                return array();
            $pairs = explode('&', $input);
            $parsed_parameters = array();
            foreach ($pairs as $pair) {
                $split = explode('=', $pair, 2);
                $parameter = OAuthUtil::urldecode_rfc3986($split[0]);
                $value = isset($split[1]) ? OAuthUtil::urldecode_rfc3986($split[1]) : '';
                if (isset($parsed_parameters[$parameter])) {
                    // We have already recieved parameter(s) with this name, so add to the list
                    // of parameters with this name
                    if (is_scalar($parsed_parameters[$parameter])) {
                        // This is the first duplicate, so transform scalar (string) into an array
                        // so we can add the duplicates
                        $parsed_parameters[$parameter] = array($parsed_parameters[$parameter]);
                    }
                    $parsed_parameters[$parameter][] = $value;
                } else {
                    $parsed_parameters[$parameter] = $value;
                }
            }
            return $parsed_parameters;
        }

        public static function build_http_query($params) {
            if (!$params)
                return '';
            // Urlencode both keys and values
            $keys = OAuthUtil::urlencode_rfc3986(array_keys($params));
            $values = OAuthUtil::urlencode_rfc3986(array_values($params));
            $params = array_combine($keys, $values);
            // Parameters are sorted by name, using lexicographical byte value ordering.
            // Ref: Spec: 9.1.1 (1)
            uksort($params, 'strcmp');
            $pairs = array();
            foreach ($params as $parameter => $value) {
                if (is_array($value)) {
                    // If two or more parameters share the same name, they are sorted by their value
                    // Ref: Spec: 9.1.1 (1)
                    natsort($value);
                    foreach ($value as $duplicate_value) {
                        $pairs[] = $parameter . '=' . $duplicate_value;
                    }
                } else {
                    $pairs[] = $parameter . '=' . $value;
                }
            }
            // For each parameter, the name is separated from the corresponding value by an '=' character (ASCII code 61)
            // Each name-value pair is separated by an '&' character (ASCII code 38)
            return implode('&', $pairs);
        }

    }

}