How do check if a PHP session is empty?

I would use isset and empty:

session_start();
if(isset($_SESSION['blah']) && !empty($_SESSION['blah'])) {
   echo 'Set and not empty, and no undefined index error!';
}

array_key_exists is a nice alternative to using isset to check for keys:

session_start();
if(array_key_exists('blah',$_SESSION) && !empty($_SESSION['blah'])) {
    echo 'Set and not empty, and no undefined index error!';
}

Make sure you're calling session_start before reading from or writing to the session array.

You can modify another users session (see below), although personally, I would recommend against it. As I imagine it can open up a whole world of session hijacking and other vulnerabilities.

With your example use case

A common user is logged, while in the same time an administrator uses the Admin functions and change some value for this user. If the value is not something obtained from the database every time, the session variable for that current logged in user need to have its value changed.

You would be better of updating the value in the database and then just checking to see if it's changed before you process the next page. If you don't want to be checking multiple user fields before each page load then when you update the user in the admin panel, you can build a hash of the values and add it to a new column called session_hash. Then just compare this field on page load

But if you still want to modify another user's session, you can set your current session_id to the targets.

// End my current session and save its id
session_start();
$my_session_id = session_id();
session_write_close();

// Modify our target session 
session_id($target_id);
session_start();
$_SESSION['is_logged_in'] = false;
session_write_close();

// Start our old session again
session_id($my_session_id);
session_start();

EDIT

Example: https://www.samdjames.uk/session_example/index.php

Example Src: https://gist.github.com/SamJUK/c220e3742487567c6262238edf85695e

session_status is available for PHP v5.4 and later, maybe that's why.

You can try with session_id :

session_id() returns the session id for the current session or the empty string ("") if there is no current session (no current session id exists).

Just what you need for PHP below 5.4 ! ;)