Encrypt/Decrypt using AES-256 openssl raw data, SHA-512 digest seed, and Null Bytes initialization vector

checksum.php

<?php
/*#############################
* Developer: Mohammad Sharaf Ali
* Designation: Sr. SE
* Version: 1.0
*/#############################

// IMPLEMENTATION
function encrypt($requestId, $validationString) {
    $digestSeed = hash('SHA512', $requestId, true);
    $seedEncArray = substr($digestSeed, 0, 32);

    $finalByteArray = openssl_encrypt(
        $validationString,
        'AES256',
        $seedEncArray,
        OPENSSL_RAW_DATA,
        str_repeat(chr(0), 16)
    );

    return base64_encode($finalByteArray);
}

function decrypt($xref, $checksum) {
    $decPwd = base64_decode($checksum);
    $digestSeed = hash('SHA512', $xref, true);
    $seedEncArray = substr($digestSeed, 0, 32);

    $finalByteArray = openssl_decrypt(
        $decPwd,
        'AES256',
        $seedEncArray,
        OPENSSL_RAW_DATA,
        str_repeat(chr(0), 16)
    );

    return $finalByteArray;
}

// USAGE
$requestId = '0000';
$validationString = 'testing123';
$encrypted = encrypt($requestId, $validationString);
echo 'Encrypted: ' . $encrypted;
echo '<br />';
$decrypted = decrypt($requestId, $encrypted);
echo 'Decrypted: ' . $decrypted;
echo '<br /><br />';