JS/Ajax : redirect data on a div with functions

I have a navigation bar with elements

<ul><li> 

the element

<li> 

elements pointing with ajax to a separate php function file

switch ($_GET['menu']){} 

with a separate function file accorded to a case (choice). actually functions are separate on a php file which is not included in my site tree. and I lose my includes when I call it. (func_nav.php)

I would like to my function file to be writted in the same file (php file containing HTML and ajax script) or previously included ?

here is the html and ajax (navbar.php)

<div id="header_menu_content_align">
<div id="header_menu_content_wrapper">
<div id="menu">


<ul class="dropdown-menu">
  <li id="uniqueId01">Home</li>
  <li id="uniqueId02">User
    <ul>
      <li id="uniqueId03">Sous-item 1</li>
      <li id="uniqueId04">Sous-item 2</li>
      <li id="uniqueId05">Sous-item 3</li>
    </ul>
  </li>
  <li id="uniqueId06">Admin</li>
</ul>


</div>
</div>  
</div>




<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script>

$('.dropdown-menu li').on('click', function(){
$.get('function/navigation/func_nav.php', { menu: this.id }, function(data){
    $('#wrapper_page_desktop_content').html(data); // inject HTML into above DIV,
});
});

</script>

here is the php file i want to include or call with all my previous includes

<?php



switch($_GET['menu']){

    case 'uniqueId01':
        menu1();
        break;
    case 'uniqueId02':
        menu2();
        break;
    case 'uniqueId03':
        menu3();
        break;
    case 'uniqueId04':
        menu4();
        break;
    case 'uniqueId05':
        menu5();
        break;
    case 'uniqueId06':
        menu6();
        break;
    default:
        someDefaultFunction();
        break;
}

function menu1(){
    include '/desktop/content/corporate01/content.php';
}
function menu2(){
    echo 'You clicked Menu USER! ';
}
function menu3(){
    echo 'You clicked Menu 3! ';
}
function menu4(){
    echo 'You clicked Menu 4! ';
}
function menu5(){
    echo 'You clicked Menu 5! ';
}
function menu6(){
    echo 'You clicked Menu 6! ';
}


?>

I would like to keep my includes => include => include => include html => include html i have some includes like SSO.php controller (desktop mobile) etc.

I would like my "get" not pointing to a file but to a included function..

$.get('function/navigation/func_nav.php', { menu: this.id }, function(data){
    $('#wrapper_page_desktop_content').html(data); // inject HTML into above DIV,
});