utils.php

    /**
     * Taken from Nikita Popov's answer on stack overflow: http://stackoverflow.com/questions/16489509/decrypting-aes-ctr-little-endian-with-php
     */
    static public function ctr_crypt($str, $cipher = MCRYPT_RIJNDAEL_256, $key, $iv) {
        $numOfBlocks = ceil(strlen($str) / 16);

        $ctrStr = '';
        for ($i = 0; $i < $numOfBlocks; ++$i) {
            $ctrStr .= $iv;

            // increment IV
            for ($j = 0; $j < 16; ++$j) {
                $n = ord($iv[$j]);
                if (++$n == 0x100) {
                    // overflow, set this one to 0, increment next
                    $iv[$j] = "\0";
                } else {
                    // no overflow, just write incremented number back and abort
                    $iv[$j] = chr($n);
                    break;
                }
            }
        }

        return $str ^ mcrypt_encrypt($cipher, $key, $ctrStr, MCRYPT_MODE_ECB);
    }