constants.php

<?php
define('SMTP_HOST', 'smtp.gmail.com');
define('SMTP_USERNAME', 'email@gmail.com');
define('SMTP_PASSWORD', '1234 abcd 2134 abcd'); // Use a secure app password, not your main email password go to https://myaccount.google.com/u/2/apppasswords this like to create app password. Don't use regular password here. and make sure you add 2 factor authentication.
define('FROM_EMAIL', 'sender@gmail.com');  // Sender email address
define('RECIPIENT_EMAIL', 'reciver@example.com'); // Recipient email address
?>