How can PHP be used to set the value of a hidden input field based on user input?

To set the value of a hidden input field based on user input in PHP, you can use a form with an input field where users can enter their data. Then, use PHP to retrieve the user input and set it as the value of the hidden input field before submitting the form.

<form action="process.php" method="post">
    <input type="text" name="user_input">
    <input type="hidden" name="hidden_input" value="<?php echo isset($_POST['user_input']) ? $_POST['user_input'] : ''; ?>">
    <input type="submit" value="Submit">
</form>