Mcrypt helper function for padding AES encryption keys to ensure PHP 5.6-7.1 is backwards compatible with PHP <= 5.5.

mcrypt-pad-aes.php

<?php
/**
 * Pad an AES encrypted key to ensure it's of valid size according
 * to specification. This function is backwards compatible with 
 * PHP <= 5.5 and is intended for usage with PHP 5.6+.
 */
function pad_aes($str) {
    $c = strlen($str);
    $validLengths = array(16, 24, 32);
    foreach ($validLengths as $l) {
        if ($c === $l) return $str;
        while ($c < $l) {
            $str .= "\0";
            $c++;
        }
    }
    return $str;
}

mcrypt-pad-usage.php

<?php
/**
 * Example of using the pad_aes() helper function to ensure your 
 * mcrypt_encrypt() and mcrypt_decrypt() functions use appropriately
 * sized key lengths.
 */

// invalid key of length 15
$key = 'abcdefghijklmni';

// our secret message for two-way encryption
$message = 'hello, world!';

// demonstration of encryption and decryption
$iv = mcrypt_create_iv(16, MCRYPT_DEV_URANDOM);
$encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, pad_aes($key), $message, MCRYPT_MODE_CBC, $iv);
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, pad_aes($key), $encrypted, MCRYPT_MODE_CBC, $iv);

// example demonstrating that the decrypted message still has padding
var_dump('SECRET: ' . $message);
var_dump('DECRYPTED WITH PADDING: ' . $decrypted);

// strip off null byte padding
$stripped = rtrim($decrypted, "\0");
var_dump('DECRYPTED: ' . $stripped);