PHP7.1 mcrypt alternative

You should use openssl_encrypt instead.

Consider using defuse or RNCryptor, they provide a complete solution, are being maintained and is correct.

For MCRYPT_RIJNDAEL_256 I posted a full answer for PHP7.3 here: https://stackoverflow.com/a/53937314/243782

snippet:

works like this with the phpseclib library

$rijndael = new \phpseclib\Crypt\Rijndael(\phpseclib\Crypt\Rijndael::MODE_ECB);
$rijndael->setKey(ENCRYPT_KEY);
$rijndael->setKeyLength(256);
$rijndael->disablePadding();
$rijndael->setBlockLength(256);

$decoded = $rijndael->decrypt($term);

echo encrypt_openssl($str, $key);

function encrypt_openssl($msg, $key, $iv = null) {
        $iv_size = openssl_cipher_iv_length('AES-256-CBC');
        if (!$iv) {
            $iv = openssl_random_pseudo_bytes($iv_size);
        }
        $encryptedMessage = openssl_encrypt($msg, 'AES-256-CBC', $key, OPENSSL_RAW_DATA, $iv);
        return base64_encode($iv . $encryptedMessage);
    }

mcrypt may be removed in PHP 7.1 alternative openssl