PHP - Prevent Duplicate Form Requests On Refresh?

Question

I have a form that makes some calculations based on the data submitted and outputs the results on the same page. I tried setting up a post redirect get script using a header 303 redirect, but that 303 only works in chrome for some reason and not other browsers like firefox or safari.

How can I prevent my form from duplicating a submission if the page is refreshed without using a header redirect?

I'm not asking for a code critique, but if you could suggest some methods to handle this properly it would be appreciated.

UPDATE

I did originally state that header redirects are not working, but I went ahead and gave it another shot as you suggested. I'm not sure where I'm going wrong w/ my logic.

If there are posts all the variables are set as sessions, the page is then redirected to clear the $_post array. When the page reloads the sessions are assigned to variables then unset and destroyed, and those variables are then passed.

But what's happening is the page reloads from the redirect and the data is either not set to the session or the sessions are being unset with the redirect as well.

Here are the two blocks of code that processes the $_POST array and sets them to sessions, and the the second block assigns the sessions to variables then unsets and destroys the sessions.

session_start();
error_reporting(E_ALL); ini_set('display_errors',1);

if(count($_POST) > 0) {
    (isset($_POST['dob-month'])? $_SESSION['dob-month'] = $_POST['dob-month']:$_SESSION['dob-month'] = null);
    (isset($_POST['dob-day'])? $_SESSION['dob-day'] = ltrim(sanitizeNumInput($_POST['dob-day']),'0'):$_SESSION['dob-day'] =  null);
    (isset($_POST['dob-year'])?$_SESSION['dob-year'] = sanitizeNumInput($_POST['dob-year']):$_SESSION['dob-year'] =null);

    $_SESSION['dob-month'] = $_POST['dob-month'];
    $_SESSION['dob-day'] = $_POST['dob-day'];
    $_SESSION['dob-year'] = $_POST['dob-year'];

        if(isset($_POST['submit'])){
    $_SESSION['submit'] = 1;}


    header("Location: /test-form/",TRUE,303);
}

if (isset($_SESSION['dob-month'])||isset($_SESSION['dob-day'])||isset($_SESSION['dob-year'])){
    (isset($_SESSION['dob-month'])? $month = $_SESSION['dob-month']:$month = null);
    (isset($_SESSION['dob-day'])? $day = ltrim(sanitizeNumInput($_SESSION['dob-day']),'0'): $day = null);
    (isset($_SESSION['dob-year'])?$year = sanitizeNumInput($_SESSION['dob-year']):$year=null);
    (isset($_SESSION['submit'])?$submit = $_SESSION['submit']: $submit = null );


    /*
        Put database-affecting code here.
    */


    session_unset();
    session_destroy();
}

Answer 1

Use this code

if (isset($_POST['submit'])) { 

//your codes
//use your current page instead of page.php
header('Location: page.php'); //clears all POST
}