JQuery $.post is not sending data to my php - AJAX problem

Question

i have a problem in my jquery code to send data using AJAX ($.post) to my php file. In my jQuery Code i want to send two variables to my php file, so i can do a sql query. I checked with alerts if there is a valid value in my variables and there is. But if I want to send them to the php they won't be recieved. In php I am using $_SESSION to check but nothing is putting out.

 $(".size-select").change(function(){
        var value = $(this).val();
        var product_id = $(this).parent().parent().siblings("input").val();
        

        if(value!== "" && product_id !== ""){
        
            $.post("article_ajax.php",{ size:value , product:product_id},function(data){

                $.each(data,function(i,v){
                    alert(i + " "+ v);
                    //$(this).siblings("color-select").append("<option value='"+v['id']+"'>"+v['desgination']+"</option>");
                    
    
                });

            },"JSON"); 
            
            
        }    
    });

and here is my php file

<?php 
session_start();

$_SESSION["test"]= $_POST;

json_encode($_SESSION["test"]);
 ?>

i hope you can help me.