salesagility_SuiteCRM/include/social/twitter/twitter_auth/twitteroauth/OAuth.php

<?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]";
    }
}

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
 */
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")
 */
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
        $privatekeyid = openssl_pkey_get_private($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
{
    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);

        $port = @$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
    }
}

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");
        }
    }

}

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)
    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);
    }
}