What potential issue arises when a user clicks "Refresh" on a page with a PHP form that submits data to a database?

When a user clicks "Refresh" on a page with a PHP form that submits data to a database, it can result in duplicate entries being inserted into the database. This is because the form data is resubmitted each time the page is refreshed. To solve this issue, you can implement a technique called Post/Redirect/Get (PRG) pattern. After the form is submitted and the data is processed, redirect the user to another page using the header() function to prevent the form data from being resubmitted if the user refreshes the page. 

// Process form data
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Insert data into the database
    
    // Redirect to another page
    header("Location: success.php");
    exit();
}

Keywords

PHP form submission database data integrity duplicate entries

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