Doesn’t Work !

I have a WP page that calls a script with following lines in it …

session_start();
$_SESSION[‘myKey’] = “Some data I need later”;
header(‘Location:https://wp.achieve-it.co.uk/test-php’);

As you can see another page called test-php is called on the last line which contains only the following lines …

<?php
print “before”;
print “myKey is set: “.$_SESSION[‘myKey’];
print “after”;
?>

The out put is ..

beforemyKey is set: after

Which means the plugin has NOT worked.

Can you help explain why this simple test of the plugin does not work?
Have I misunderstood what the plugin is about?

Many Thanks in advance.