How to create two way encode/decode methods using use-specific key in PHP

encode-decode.php

<?php

$key = "secret_key";
$data ="String";

// Encodeing
$encoded = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($key), $data, MCRYPT_MODE_CBC, md5(md5($key))));

// Decoding
$decoded = rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, md5($key), base64_decode($encoded), MCRYPT_MODE_CBC, md5(md5($key))), "\0");


// Ie.

echo $encoded;
echo "<br>";
echo $decoded;