使用jquery ajax与php时的SESSION问题

Question

I have my first script and when I click the forward button (.dev1) the next tab opens and a session is created with the value of former tab's selection.

If I want to change the selection; there is a back button (.ger1). It returns back to the first tab, and unset the session to have a new value (i checked it, there is no value).

After I change the selection and click the next button, the session has the first value again, not the new selection's value, although it hasn't any value after deleting. It requires refreshing the page on the first tab, then selecting the new selection.

//script to create session:

$(".dev1").click(function () {
    var action = "add";
    var kesim1 = $("#resimilk").data("kesim1");

    $.ajax({
        url: "action.php",
        method: "POST",
        data: {
            kesim1: kesim1,
            action: action
        },
    });
});

//script to delete session:

$(".ger1").click(function () {
    var action = "kesimsil";
    var kesim;

    $.ajax({
        url: "action.php",
        method: "POST",
        data: {
            kesim: kesim,
            action: action
        },
    });
});

action.php

if($_POST["action"] == "add"){

    $kesim1=$_POST["kesim1"];

    $_SESSION['kesim']=$kesim1;

    echo $kesim1;

}
if($_POST["action"] == "kesimsil") {

    $kesim=$_POST["kesim"];

    unset($_SESSION['kesim']);

    echo $kesim;

}