How can hidden form fields in PHP cause unexpected behavior and how can this issue be avoided?

Hidden form fields in PHP can cause unexpected behavior when they are manipulated by users through tools like browser developer tools. To avoid this issue, it is recommended to use server-side validation and not rely solely on hidden form fields for sensitive data or logic. Additionally, using session variables or server-side storage can help prevent manipulation of hidden form fields.

// Avoid using hidden form fields for sensitive data or logic
// Use server-side validation and session variables instead

// Example of using session variables to store sensitive data
session_start();
$_SESSION['user_id'] = 123;

// Retrieving the user_id from session
$user_id = $_SESSION['user_id'];