How can the issue of users being able to resubmit entries using the back button in PHP forms be prevented?

Issue: Users can resubmit entries by using the back button in PHP forms, causing duplicate form submissions and potentially affecting data integrity. To prevent this, we can implement a token-based solution where a unique token is generated for each form submission and checked to ensure that the form is only submitted once. PHP Code Snippet: 

```php
session_start();

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    if (empty($_POST['token']) || $_POST['token'] !== $_SESSION['token']) {
        // Invalid token, do not process the form submission
        exit('Invalid form submission');
    }
    
    // Process the form data
    
    // Generate a new token to prevent resubmission
    $_SESSION['token'] = bin2hex(random_bytes(32));
}
```

In your form HTML:
```html
<form method="post" action="submit.php">
    <input type="hidden" name="token" value="<?php echo $_SESSION['token']; ?>">
    <!-- Other form fields here -->
    <button type="submit">Submit</button>
</form>
```

This code snippet generates a unique token using `random_bytes()` and stores it in the session. On form submission, the token is checked to ensure it matches the one stored in the session. If the tokens do not match, the form submission is considered invalid. Additionally, a new token is generated after each valid form submission to prevent duplicate submissions.

Keywords

PHP form submission session management header redirection prevent resubmission

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