AJAX会话PHP错误

Question

I have been performing session creation using PHP. I'm trying to create session using AJAX call.

This is my ajax call code :

var xhr = new XMLHttpRequest();
xhr.open('POST', 'https://www.vedarjana.com/signin.php');
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.onload = function() {
    console.log('Signed in as: ' + xhr.responseText);
};
xhr.send('id=' + "123");
xhr.onreadystatechange = function() {
    if (xhr.readyState == XMLHttpRequest.DONE) {
        if (xhr.responseText == '') {
            //alert(xhr.responseText);

        } else {

            location.reload();
        }
    }
}

Now This is my PHP code for session generation :

session_start();
$_SESSION["frontUserIslogin"] = true;
$_SESSION["frontUserId"] = "123";
$_SESSION["frontUserName"] = "abc";
$_SESSION["frontUserProfilePic"] = "abc.png";
echo $_SESSION["frontUserName"];

Now when I perform AJAX call, session is generating as its returning username. But when page reloads, I'm checking var_dump($_SESSION) then its showing NULL

Please help me I'm stuck on it from last 3 weeks

Thanks in advance