<?php
// vim: foldmethod=marker

/* Generic exception class
 */
if ( ! class_exists( 'OAuthException' ) ) {
  class OAuthException extends Exception {
    // pass
  }

  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]";
    }
  }

  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
   */
  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);

      // Check for zero length, although unlikely here
      if (strlen($built) == 0 || strlen($signature) == 0) {
        return false;
      }

      if (strlen($built) != strlen($signature)) {
        return false;
      }

      // Avoid a timing leak with a (hopefully) time insensitive compare
      $result = 0;
      for ($i = 0; $i < strlen($signature); $i++) {
        $result |= ord($built{$i}) ^ ord($signature{$i});
      }

      return $result == 0;
    }
  }

  /**
   * 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")
   */
  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")
   */
  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")
   */
  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;
    }
  }

  class OAuthRequest {
    protected $parameters;
    protected $http_method;
    protected $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) {
      $parameters = ($parameters) ? $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';
      $http_url = ($http_url) ? $http_url : $scheme .
                                '://' . $_SERVER['SERVER_NAME'] .
                                ':' .
                                $_SERVER['SERVER_PORT'] .
                                $_SERVER['REQUEST_URI'];
      $http_method = ($http_method) ? $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 ($http_method == "POST"
            &&  isset($request_headers['Content-Type'])
            && 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 (isset($request_headers['Authorization']) && substr($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) {
      $parameters = ($parameters) ?  $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);

      $scheme = (isset($parts['scheme'])) ? $parts['scheme'] : 'http';
      $port = (isset($parts['port'])) ? $parts['port'] : (($scheme == 'https') ? '443' : '80');
      $host = (isset($parts['host'])) ? strtolower($parts['host']) : '';
      $path = (isset($parts['path'])) ? $parts['path'] : '';

      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
    }
  }

  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 instanceof OAuthRequest
          ? $request->get_parameter("oauth_signature_method")
          : NULL;

      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 instanceof OAuthRequest
          ? $request->get_parameter("oauth_consumer_key")
          : NULL;

      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 instanceof OAuthRequest
           ? $request->get_parameter('oauth_token')
           : NULL;

      $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 instanceof OAuthRequest
          ? $request->get_parameter('oauth_timestamp')
          : NULL;
      $nonce = $request instanceof OAuthRequest
          ? $request->get_parameter('oauth_nonce')
          : NULL;

      $this->check_timestamp($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");
      }
    }

  }

  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
    }

  }

  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)
    // May 28th, 2010 - method updated to tjerk.meesters for a speed improvement.
    //                  see http://code.google.com/p/oauth/issues/detail?id=163
    public static function split_header($header, $only_allow_oauth_parameters = true) {
      $params = array();
      if (preg_match_all('/('.($only_allow_oauth_parameters ? 'oauth_' : '').'[a-z_-]*)=(:?"([^"]*)"|([^,]*))/', $header, $matches)) {
        foreach ($matches[1] as $i => $h) {
          $params[$h] = OAuthUtil::urldecode_rfc3986(empty($matches[3][$i]) ? $matches[4][$i] : $matches[3][$i]);
        }
        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)
          // June 12th, 2010 - changed to sort because of issue 164 by hidetaka
          sort($value, SORT_STRING);
          foreach ($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);
    }
  }
}