rc4 algorithm in php

rc4.php

<?php

class RC4
{
    public static $_key = 'aRand0mStr1ng';

    public static function encode($string, $key = null)
    {
        $res = self::_rc4($string, $key);
        $encrypted = unpack('H*', $res);
        if (!empty($encrypted)) return array_values($encrypted)[0];
        return '';
    }

    public static function decode($string, $key = null)
    {
        if (ctype_xdigit($string)) {
            $str = pack('H*', $string);
            return self::_rc4($str, $key);
        } else {
            return '';
        }
    }

    public static function _rc4($str, $key = null)
    {
        if(!$key) $key = self::$_key;
        $s = array();
        for ($i = 0; $i < 256; $i++) {
            $s[$i] = $i;
        }
        $j = 0;
        for ($i = 0; $i < 256; $i++) {
            $j = ($j + $s[$i] + ord($key[$i % strlen($key)])) % 256;
            $x = $s[$i];
            $s[$i] = $s[$j];
            $s[$j] = $x;
        }
        $i = 0;
        $j = 0;
        $res = '';
        for ($y = 0; $y < strlen($str); $y++) {
            $i = ($i + 1) % 256;
            $j = ($j + $s[$i]) % 256;
            $x = $s[$i];
            $s[$i] = $s[$j];
            $s[$j] = $x;
            $res .= $str[$y] ^ chr($s[($s[$i] + $s[$j]) % 256]);
        }
        return $res;
    }
}