protected function generatePbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)
    {
        $algorithm = strtolower($algorithm);
        if(!in_array($algorithm, hash_algos(), true))
            trigger_error('PBKDF2 ERROR: Invalid hash algorithm.', E_USER_ERROR);
        if($count <= 0 || $key_length <= 0)
            trigger_error('PBKDF2 ERROR: Invalid parameters.', E_USER_ERROR);

        if (function_exists("hash_pbkdf2")) {
            // The output length is in NIBBLES (4-bits) if $raw_output is false!
            if (!$raw_output) {
                $key_length = $key_length * 2;
            }
            return hash_pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output);
        }

        $hash_length = strlen(hash($algorithm, "", true));
        $block_count = ceil($key_length / $hash_length);

        $output = "";
        for($i = 1; $i <= $block_count; $i++) {
            // $i encoded as 4 bytes, big endian.
            $last = $salt . pack("N", $i);
            // first iteration
            $last = $xorsum = hash_hmac($algorithm, $last, $password, true);
            // perform the other $count - 1 iterations
            for ($j = 1; $j < $count; $j++) {
                $xorsum ^= ($last = hash_hmac($algorithm, $last, $password, true));
            }
            $output .= $xorsum;
        }

        if($raw_output)
            return substr($output, 0, $key_length);
        else
            return bin2hex(substr($output, 0, $key_length));
    }