session_start doesnt stick in ajax call

[sheeview]

im trying to get PHP operate well with javascript. I have looked on internet and put all the solution inside at once but none work. I want to create a cross-site AJAX request but i cant seem to be able to have session_start() stick.

I have this PHP code on server:

<?php

    header("Content-Type: application/json");
    header("Access-Control-Allow-Credentials: true");
    header("Access-Control-Allow-Origin: null");

    if (isset($_SESSION))
    {
        $_SESSION["time"] = time();
        if (isset($_SESSION["time"])) echo json_encode("Session exists:" .$_SESSION["time"]);   
    }
    else
    {
        echo json_encode("Session does not exist!");
        session_start();
    }

?>

I have this JavaScript(jQuery) code on client:

$(function()
{
    $(".action").on("click", call);
});

function call()
{
    $.ajax({
        url: "http://localhost/api.php",
        method: "POST",
        data: [],
        async: true,
        dataType: "json",
        xhrFields: { withCredentials: true },
        crossDomain: true,
        processData: true,
        headers: {"accept": "application/json"},
        cache: true,
        success: function(result)
        {
            console.log(result);
        },
        error: function(e)
        {
            console.error(e);
        }
    });
}

No matter how many times i push the button i get "session does not exist"

Is there a way I can let PHP and jQuery establish safe session between each other and sort all the necessary session cookies by themselves? I can't use <?php var_dump($_SESSION); ?> because not every user will be a website.

[pzuurveen]

$_SESSION gets set by session_start()
before that is empty
So:

<?php
    session_start();
    header("Content-Type: application/json");
    header("Access-Control-Allow-Credentials: true");
    header("Access-Control-Allow-Origin: null");

    if (isset($_SESSION))
    {
        $_SESSION["time"] = time();
        if (isset($_SESSION["time"])) echo json_encode("Session exists:" .$_SESSION["time"]);   
    }
    else
    {
        echo json_encode("Session does not exist!");
    }

[Dani]

Sheeview, did that work?

[Stelian_1]

<?php
session_id(); // to set session id
session_name(); // to set the session name
session_start(); // start the session before $_SESSION otherwise there is no $_SESSION not even a NULL one
isset($_SESSION);

[Stelian_1]

So as for your code:

<?php
session_name('timer'); // use a name or id for extra features
session_start();
header("Content-Type: application/json");
header("Access-Control-Allow-Credentials: true");
header("Access-Control-Allow-Origin: null");
// to verify the session after session start use $timesession = session_name(); // retrives the name of the session
$_SESSION['time'] = time();
if (isset($_SESSION["time"])) {
    echo json_encode("Session exists:" .$_SESSION["time"]);   
} else
{
    echo json_encode("Session does not exist!");
}
// if you need extra sessions or you want to stop the session and resume it another time or on other page or script
session_write_close();
// and to retrive it elswere
session_name('timer'); // set its name
session_start(); // resumes the session
$last_time = $_SESSION['time']; // get the value you had