Set Session variable using javascript in PHP

Not possible. Because JavaScript is client-side and session is server-side. To do anything related to a PHP session, you have to go to the server.

I solved this question using Ajax. What I do is make an ajax call to a PHP page where the value that passes will be saved in session.

The example that I am going to show you, what I do is that when you change the value of the number of items to show in a datatable, that value is saved in session.

$('#table-campus').on( 'length.dt', function ( e, settings, len ) {
    $.ajax ({
       data:        {"numElems": len},
       url:        '../../Utiles/GuardarNumElems.php',
       type:        'post'
    }); 
});

And the GuardarNumElems.php is as following:

<?php    
    session_start();

    if(isset ($_POST['numElems'] )){
        $numElems = $_POST['numElems'];        
        $_SESSION['elems_table'] = $numElems;
    }else{
        $_SESSION['elems_table'] = 25;
    } 
?>