PHP Post–Redirect–Get (PRG) – Prevent Duplicate Form Submissions

Implement the PRG pattern in PHP to enhance UX and avoid form resubmission on page refresh.

Introduction – Why PRG Matters in PHP Forms

If a user submits a form and then refreshes the page, most browsers will warn that resubmitting the form could lead to duplicate entries. This happens when a form submitted via POST is re-sent on refresh. The Post–Redirect–Get (PRG) pattern is a proven solution to this common problem.

In this guide, you’ll learn:

• What PRG is and why it’s useful
• How to implement PRG in PHP
• How PRG improves UX and data consistency
• Best practices for using PRG in form handling

PHP Post–Redirect–Get (PRG)

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    // Process form data here
    header("Location: thank-you.php");
    exit;
}

PRG prevents form resubmission if the user refreshes the page after a POST request.
Use header("Location: ...") after processing and before outputting anything.

How PRG Works – Step-by-Step

1. POST: User submits a form → PHP processes the data
2. Redirect: PHP sends a Location: header to a new page (GET request)
3. GET: User lands on a new URL with a fresh state (no form data)

PRG Implementation Example

Form Page (form.php)

<form action="form.php" method="post">
  <input name="username">
  <input type="submit" value="Submit">
</form>

PRG Logic (form.php)

session_start();

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $_SESSION['flash'] = "Form submitted successfully!";
    header("Location: form.php");
    exit;
}

if (isset($_SESSION['flash'])) {
    echo "<p>" . $_SESSION['flash'] . "</p>";
    unset($_SESSION['flash']);
}

This pattern:

• Processes the form
• Redirects the user
• Displays a one-time flash message

Why Use PRG?

Common Mistakes to Avoid

• Echoing output before calling header("Location:")
• Not calling exit after redirect — it can cause unexpected behavior
• Forgetting to unset session flash messages after displaying

Summary – Recap & Next Steps

The PRG pattern is a best practice in PHP development, especially for handling form submissions. It enhances UX, prevents duplicate data, and enables flash messages for better user feedback.

Key Takeaways:

• PRG = Post → Redirect → Get
• Use it to prevent form resubmission on refresh
• Combine with sessions for one-time flash messages
• Always call exit after header("Location:")

Real-World Use Cases:
User registration, contact forms, checkout flows, login redirects, profile updates

Frequently Asked Questions (FAQs)

What does PRG stand for in PHP?
Post–Redirect–Get. It’s a design pattern used to prevent duplicate form submissions.

Why use header("Location: ...") after POST?
It sends the browser to a new page, clearing POST data from memory to prevent re-submission.

Do I need to call exit after redirect?
Yes. To stop PHP from continuing script execution after the header is sent.

Can I use PRG with flash messages?
Absolutely. Store temporary messages in $_SESSION and display them after redirect.

Is PRG useful for AJAX forms?
Not directly. PRG is for traditional page-based forms. Use JSON response handling for AJAX.