How to include php with JQuery?

I have been struggling with this for a while now.

What I found is this:

<div id="include"></div>
        <script>
$(function(){
    $("#include").load("/test.php");
});
    </script>   

This does work when in test.php is a simple code like:

<?php echo 'echo world'; ?>

But it doesn't work when I want to grab Wordpress variables.

<?php   $current_user_id =  $current_user->ID;
echo $current_user_id; ?>

There's nothing outputed. No error. Just a blank.

It's like it doesn't send the variables to the test.php. So test.php doesn't echo values filled. When I use <?php include 'test.php';?> it does fill the variables and works as I wanted.

So basically, .load simply loads the file.

But I want <?php include;?> to be executed instead of .load.

When I use <?php include;?> I can grab variables, which I need.

Is there other command instead of .load to do that?

Any help would be appreciated.