How can PHP variables be stored in sessions to control form submission behavior and prevent duplicate entries in a database?

To prevent duplicate form submissions and entries in a database, we can store a unique token in a session variable when the form is submitted. This token can be checked before processing the form data to ensure that the submission is not a duplicate. If the token matches, the form data can be processed; otherwise, the submission can be ignored. 

```php
session_start();

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    if (!isset($_SESSION['form_token'])) {
        $_SESSION['form_token'] = md5(uniqid(rand(), true));
    } else {
        if ($_POST['form_token'] == $_SESSION['form_token']) {
            // Process form data
            // Prevent duplicate entries in the database
            // Reset form token after processing
            unset($_SESSION['form_token']);
        } else {
            // Duplicate form submission detected
            // Handle accordingly
        }
    }
}
```
In this code snippet, we generate a unique token when the form is submitted and store it in a session variable. We then check if the submitted token matches the one stored in the session. If they match, we process the form data and prevent duplicate entries in the database. Finally, we reset the form token after processing the form data.

Keywords

PHP sessions variables form submission database duplication

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