How to post an extra value when submitting a form in PHP?

one text

Solution:

Yes you can, using a hidden input:

<body>
    <form action="sendMail.php" method="post">
        <input type="hidden" name="the-name-you-need" value="<?php echo $id = $_GET["idPayment"]; ?>">
        ...
    </form>

</body>

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