将javascript变量转换为php变量

Question

I am trying to use a global variable from this code in javascript:

function myFunction(X) 
            {
                    index = X.selectedIndex ;
                    alert(X.options[index].value) ;

            }

to this other file in the same project but in php:

function Trier(){

    $eleve = new Eleve();
    $matiere = new Matiere();
    $examen = new Examen();

    if($_GET['classe'] == "Tous" && $_GET['exam'] == "Aucun"){
        listEleveGlobal();  
}

the final objectif is to replace $_GET['classe'] by the variable 'y' thinks!