Mengambil data dari localtstorage ke ajax

Saya mau mengambil data dari localStorage dengan mengeksekusi ke php menggunakan ajax, tapi kok gk jalan ya??

Kode ajax

<script>
    $(document).ready(function(){
        var b = localStorage.getItem('subjek');
        if(b!==null){
        function notif_update(){
                $.ajax({
                    url:"update_notif.php",
                    data: {'subjek':b},
                    dataType: 'html',
                    type:'POST'
                });
            }

        }
    });
    </script>

Kode PHP update_notif.php

<?php
session_start();
$subjek = $_POST['subjek'];
$rel = "../../training_notif/status/".$_SESSION['username']."/status.data";
$cTime = date("Y-m-d H:i:s");
$Handle = fopen($rel,"a");
$txt = '{"username":"'.$_SESSION['username'].'","subjek":"'.$subjek.'","datetime":"'.$cTime.'"}' ;
$write = fwrite($Handle,$txt . chr(13) . chr(10)) ;
fclose($Handle) ;
echo "<script>localStorage.removeItem('subjek');</script>";
?>

Local Storagenya bentuknya: subjek : lala (key) : (value)