Question

I got a POST form, and it does send the data to the same file, and if use hit the back button in his browser, he can easily just re-send the data, and it’ll be still read.

Is there any way to avoid such behaviour?

Answer 1

The link Levi sent will answer for you. But in case you want an alternative, here is how I do it…

User posts to a class, like yours. same file. In the beginning of the class I do post processing. For this example I will make it very simple…

<?php
session_start();

//set form vars ahead of time so you can pre-populate the value attr on post
$form = array(
    'name' => '',
    'email' => ''
);

if(!empty($_POST))
{
    //do some kind of validation...
    $errors = array();
    if(trim($_POST['name']) == '')
        $errors[] = 'Please enter your name';

    if(empty($errors))
    {
        $_SESSION['message'] = 'Thank you for participating';
        header('location: /form.php'); // same file
        exit;
    }
    else
    {
        // set the form vars to the post vars so you don't lose the user's input
        $form['name'] = $_POST['name'];
        $form['email'] = $_POST['email'];

        $message = '<span style="color:red">';
        foreach($errors AS $error)
        {
            $message .= $error."<br />";
        }
        $message .= '</span>';
        $_SESSION['message'] = $message;
    }
}

if(isset($_SESSION['message']))
{
    echo $_SESSION['message'];
    unset($_SESSION['message']);
}
?>
<form id="some_form" action="" method="post">
    <fieldset>
        <label for="name">Name</label> <input type="text" name="name" value="<?php echo $form['name']; ?>" />
        <br /><br />
        <label for="email">Email</label> <input type="text" name="email" value="<?php echo $form['email']; ?>" />
        <br /><br />
        <input type="submit" name="submit" value="Submit" />
    </fieldset>
</form>

Now you can refresh over and over and not submit the form twice.

Edited to show further example. Obviously your validation and error handling should be a bit more sophisticated than this, but this should get you in the right direction.