php - 在外部页面上使用session的变量

I need to check sessions variable on external host, e.g.
Source Site (using Laravel):

session([
    'myVar' => 'valueOfThisVar'
]);

Destination Site:
I'm dumping $_SESSION but I cannot see this variable.
I have tried with cookie too:

setcookie('myVar', 'valueOfThisVar', time()+3600);  

and dumped $_COOKIE but it's same effect - I cannot access this variable.
Am I doing sth wrong?