PHP Session Variables Not Preserved with Ajax

PHP session variables not preserved with ajax

I think you're missing session_start() on the page that Ajax calls.

You need:

<?php
session_start();
if(isset($_SESSION['views']))
    { echo "Views: " . $_SESSION['views'];} 
    else 
    { echo "Views: NOT SET";}
?>

Cannot Keep Ajax Preserved Session Variables

There are few issues with your code, such as:

Normal code flow: When you first visit index.php, it will trigger the AJAX request, and will subsequently set the session variable; so that when you visit update.php page, you'll get the desired result.
Your code flow: Having said the above point, if you directly visit the update.php page without visiting index.php in the first place, you'll get this error,

Notice: Undefined index: numSum in ...
And that's because $_POST['numSum'] is not set, in fact, the entire superglobal $_POST array is empty.
So the solution is this,
Wrap this statement $_SESSION['numSum1'] = $_POST['numSum']; inside an if block, like this:
```
if(!isset($_SESSION['numSum1']) || empty($_SESSION['numSum1'])){
    $_SESSION['numSum1'] = isset($_POST['numSum']) ? $_POST['numSum'] : array();
}
```

There's also a small syntax error in your code,

function atest() {

    var id_toc = <?php echo json_encode($_SESSION['numSum1']); ?>;
    window.alert(id_toc);
{   <============ See here, it should be }

You forgot to add a closing parentheses }

Finally, from the documentation,

... Session data path. If specified, the path to which data is saved will be changed. session_save_path() needs to be called before session_start() for that purpose.

Edited:

Change your if block in the following way,

if(!isset($_SESSION['numSum1']) || empty($_SESSION['numSum1']) || (isset($_POST['numSum']) && $_POST['numSum'] != $_SESSION['numSum1'])){
    $_SESSION['numSum1'] = isset($_POST['numSum']) ? $_POST['numSum'] : array();
}

Also, learn about comparison operators in PHP, especially about ternary operator. The question mark(?:) you're talking about is related to ternary operator. Here are the necessary references,

http://php.net/manual/en/language.operators.comparison.php (Comparison operators)
http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary (Ternary operator)

php session not working with ajax

According with the official documentation, you must call the session_start() method in each one of pages where you are going to use the session, so try to call session_start() in your index.php.

This method starts new or resumes existing session, also check your PHPSESSID cookie sent with the AJAX request that match with your index.php PHPSESSID.