PHP：在ajax请求中呈现包含的文件

I have the following files

 /includes
     - file_inc1.php
     - file_inc2.php
 ajax.php
 index.php

index.php calls an ajax request to ajax.php and ajax.php has something like below code

$nextStep = $_GET["nextStep"];
if($nextStep == 1) {
  include "includes/file_inc1.php";
} else if($nextStep == 2) {
  include "includes/file_inc2.php";
}

With this set-up, does anyone have a clue why the returned response from ajax is empty?

$nextStep = $_GET["nextStep"];
if($nextStep == '1') {
  include "includes/file_inc1.php";
} else if($nextStep == '2') {
  include "includes/file_inc2.php";
}

Thanks for all your responses. I was able to solve my own problem. I need to make sure that on my includes/file_inc1.php and includes/file_inc2.php files, I use ob_start() and ob_get_clean() like so:

<?php
   ob_start();
   // HTML + MYSQL + OTHERS
   echo ob_get_clean();
?>