<?php

$plaintext = 'zzzzzz';
$key = str_repeat('k', 32);
$nonce = str_repeat('n', 12);
$aad = '';

$r = openssl_encrypt(
    $plaintext,
    'aes-128-gcm',
    $key,
    OPENSSL_RAW_DATA,
    $nonce,
    $newtag,
    $aad
);

echo bin2hex($r);