如何将特定页面作为ajax加载屏幕添加到网站？

I want to add a PHP page with custom ajax loading design as ajax loader to website pages. I'm currently using very simple code in below and I need your help to guide me how to add my PHP page in this loading screen code.

Here is JavaScript code:

window.addEventListener("load", function(){
    var load_screen = document.getElementById("load_screen");
    document.body.removeChild(load_screen);
});

and

<div id="load_screen"> <div id="loading">loading document </div></div>

The above code currently is working well, but it's only text, I need to show ajax.php instead of that loading screen text.

actually I did like this, but both pages load together, I mean ajax.php and main page will load together, it should be ajax.php first then after main page will load, but I don't know why both of them load together, what is the wrong with my code?

<div id="load_screen"> <div id="loading"><?php include "ajax.php"; ?></div></div>

1: How can I add ajax.php instead of loading document text ?

2: How to add timer for this loading?

3: Why ajax.php and main page (both) load together? It should be ajax.php load first then main page will load. so how to solve it?

Thank you