In addition to mcrypt you could use phpseclib, a pure PHP AES implementation. eg
<?php
include('Crypt/AES.php');
$cipher = new Crypt_AES(CRYPT_AES_MODE_ECB);
$cipher->setKey('abcdefghijklmnopijklmnop');
$size = 10 * 1024;
$plaintext = str_repeat('a', $size);
echo $cipher->decrypt($cipher->encrypt($plaintext));
?>