How do I add $_SESSION[''] to my wordpress page?

Question

I am trying to take my existing object array then json_encode it and add it to a $_SESSION[''] so I can request it on other pages on my website. Is there a way to save my session string (or array) and display it on another page?

Below is the code of the array I am trying to add into a session. ( I don't know if I need to encode it, but I thought it would increase performance perhaps?)

    $postArray = array(
        "CompanyID" => "5",
        "FirstName" => $_POST['first_name'],
        "LastName" => $_POST['last_name'],
        "Email" => $_POST['email'],
        "Company" => $_POST['company'],
        "Phone" => $_POST['phone'],
        "Fax" => $_POST['fax'],
        "AdressLine1" => $_POST['address'],
        "AdressLine2" => "",
        "City" => $_POST['city'],
        "DistrictID" => $_POST['state'],
        "CountryID" => $_POST['country'],
        "PostCode" => $_POST['postcode'],
        "SpecialInstructions" => $_POST['special'],
        "Items" =>
                    $item_array
        ,
        "Source" => "Web submission"
    );
    $json = json_encode($postArray);

Then right after that code I try to initialize an action to wp to start and add to session

add_action('wp', 'start_my_session');
function start_my_session() {
    session_start();
    $_SESSION['order_details'] = $GLOBALS['json'];
}

After that I try to call it on a new page with this code

<?php
add_action('wp_footer', 'show_session_var');
function show_session_var() {
    if(isset($_SESSION['order_details'])) echo $_SESSION['order_details'];
}
?>

When I try to retrieve the data I get this error: session_start(): Cannot start session when headers already sent but I need to create a session because wordpress doesn't automatically make one?

Any thoughts? I am new to php development in Wordpress.

Answer 1

Remove your session_start() and at the beginning of your functions.php put this:

if (!session_id()) {
    session_start();
}

For use from a plugin use this:

function register_session(){
    if( !session_id() )
        session_start();
}
add_action('init','register_session');

Answer 2

There's absolutely no need to do all of this of the answer above, simply use session_start() at the very top of your script, that will either retrieve the existing session, or start a new one if no one exists, all by itself (check here). You simply need to call it at the very top of the script wherever you use $_SESSION content. So, use session_start() at the top of the script where you insert whatever into your $_SESSION superglobal, and then also call session_start() at the very top of the script where you wanna use it. works always, and gets particularly easy when you use page templates and custom themes. Also, adding session initiations into your functions.php file will open sessions on ANY of your pages, not just the ones where you want that, so you may do more than you would actually need. You can also simply call session_start() in the corresponding plugin pages / scripts, etc.