try "POST",

            url: "/ajax_add_selected_product_to_session.php?global_check",

            success: function(msg){

                /* add here what to do on success */

                //alert(msg);

                howmanyfromsession = msg;

                if (howmanyfromsession == 1){

                    $(".amount").empty();

                    $(".amount").append(' (Derzeit ist ' + howmanyfromsession + '