php 加密解密字符串

/*********************************************************************
函数名称:encrypt
函数作用:加密解密字符串
使用方法:
加密     :encrypt('str','E','nowamagic');
解密     :encrypt('被加密过的字符串','D','nowamagic');
参数说明:
$string   :需要加密解密的字符串
$operation:判断是加密还是解密:E:加密   D:解密
$key      :加密的钥匙(密匙);
 *********************************************************************/
    function encrypt($string, $operation, $key = 'WMqsfPpS9hwyoJnFP')
    {
        $key = md5($key);
        $key_length = strlen($key);
        $string = $operation == 'D' ? base64_decode($string) : substr(md5($string . $key), 0, 8) . $string;
        $string_length = strlen($string);
        $rndkey = $box = array();
        $result = '';
        for ($i = 0; $i <= 255; $i++) {
            $rndkey[$i] = ord($key[$i % $key_length]);
            $box[$i] = $i;
        }
        for ($j = $i = 0; $i < 256; $i++) {
            $j = ($j + $box[$i] + $rndkey[$i]) % 256;
            $tmp = $box[$i];
            $box[$i] = $box[$j];
            $box[$j] = $tmp;
        }
        for ($a = $j = $i = 0; $i < $string_length; $i++) {
            $a = ($a + 1) % 256;
            $j = ($j + $box[$a]) % 256;
            $tmp = $box[$a];
            $box[$a] = $box[$j];
            $box[$j] = $tmp;
            $result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));
        }
        if ($operation == 'D') {
            if (substr($result, 0, 8) == substr(md5(substr($result, 8) . $key), 0, 8)) {
                return substr($result, 8);
            } else {
                return '';
            }
        } else {
            return str_replace('=', '', base64_encode($result));
        }
    }