http://php.net/manual/en/function.mcrypt-encrypt.php

func.mcrypt.php

<?php
function encrypt($plaintext,$plainkey){
    $key = pack('H*', "$plainkey");

    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

    $plaintext_utf8 = utf8_encode($plaintext);

    $ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key,
        $plaintext_utf8, MCRYPT_MODE_CBC, $iv);

    $ciphertext = $iv . $ciphertext;

    $ciphertext_base64 = base64_encode($ciphertext);

    return $ciphertext_base64;
}

function decrypt($ciphertext_base64,$plainkey)
{
    $key = pack('H*', "$plainkey");

    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);

    $ciphertext_dec = base64_decode($ciphertext_base64);
    $iv_dec = substr($ciphertext_dec, 0, $iv_size);

    $ciphertext_dec = substr($ciphertext_dec, $iv_size);

    $plaintext_utf8_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key,
        $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);

    return $plaintext_utf8_dec;
}

test.mcrypt.php

<?php
include "func.mcrypt.php";

$key = "12345";
$text = "This string was AES-256 / CBC / ZeroBytePadding encrypted.";

$ciphertext = encrypt($text,$key);
$retext = decrypt($ciphertext,$key);

echo "$text\n$ciphertext\n$retext";