PHP Session Variables not updating even on same page – CPanel

From my web page I want to write (async via ajax) to an SQL table, using a generated value as a unique key. Later (could be 1 minute or 1 day, but within the same browser session) from another page, I want to be able to update that SQL record. I could pass the key value in the url but prefer to use session variables. To avoid the possibility that another web page destroys the session I am trying to use a custom cookie (not PHPSESSID), but the session variables aren’t updating.

To simplify, I took the code out of ajax and ran it direct from my browser. There are no errors or messages.

Here are the two pieces of code: page1.php

    <?php
    if(isset($_COOKIE["gcpp_imp"])){$impressionId=$_COOKIE["gcpp_imp"];}
    else{$impressionId="Imp".substr(microtime(),11,11).substr(microtime(),1,7);
    setcookie("gcpp_imp", $impressionId, 0, "/");}
    session_name("gcpp_imp");
    session_id($impressionId);
    session_start();
    $_SESSION["Test"] = "Hello";
    if(isset($_SESSION["Number"])){
        $_SESSION["Number"] = $_SESSION["Number"]+1;
    }
    else{
        $_SESSION["Number"] = 1;
    }
    echo "PAGE 1<br>";
    echo "session_save_path = ".session_save_path()."<br>";
    if (is_writable(session_save_path())) {
        echo "Session path '".session_save_path()."' is writable for PHP!<br>"; 
    }
    else {
        echo "Session path '".session_save_path()."' is not writable for PHP!<br>"; 
    }
    echo "session_name = ".session_name()."<br>";
    echo "session_id = ".session_id()."<br>";
    echo "session_status = ".session_status()."<br>";
    echo "session_Test = ".$_SESSION["Test"]."<br>";
    echo "session_Number = ".$_SESSION["Number"]."<br>";
    ?>

and page2.php

    <?php
    session_name("gcpp_imp");
    session_id($_COOKIE["gcpp_imp"]);
    session_start();
    echo "PAGE 2<br>";
    echo "session_save_path = ".session_save_path()."<br>";
    if (is_writable(session_save_path())) {
        echo "Session path '".session_save_path()."' is writable for PHP!<br>"; 
    }
    else {
        echo "Session path '".session_save_path()."' is not writable for PHP!<br>"; 
    }
    echo "session_name = ".session_name()."<br>";
    echo "session_id = ".session_id()."<br>";
    echo "session_status = ".session_status()."<br>";
    echo "session_Test = ".$_SESSION["Test"]."<br>";
    echo "session_Number = ".$_SESSION["Number"]."<br>";
    ?>

Output from page1.php

PAGE 1
session_save_path = /var/cpanel/php/sessions/ea-php56
Session path '/var/cpanel/php/sessions/ea-php56' is writable for PHP!
session_name = gcpp_imp
session_id = Imp1568485583.309708
session_status = 2
session_Test = Hello
session_Number = 1

Output from page2.php

PAGE 2
session_save_path = /var/cpanel/php/sessions/ea-php56
Session path '/var/cpanel/php/sessions/ea-php56' is writable for PHP!
session_name = gcpp_imp
session_id = Imp1568485583.309708
session_status = 2
session_Test =
session_Number =

The pages are both run from https, with no subdomain, from the same folder. My server is a VPS shared hosting at Hostiso, running PHP Version 5.6.40.

phpinfo.php shows session.gc_divisor = 0, session.gc_maxlifetime = 2880 (I will want to increase this) and session.gc_probability = 0.

When I refresh page1.php the output stays the same. So the cookie gcpp_imp is being set once and once only, the key/session_id $impressionId is being generated once and once only, and I can see the cookie and value in my browser. But $_SESSION[“Number”] is not being updated in page1.php and neither session variable is being displayed in page2.php.

What am I missing?