Why is PHP session_destroy() not working?

If you need to clear the values of $_SESSION, set the array equal to an empty array:

$_SESSION = array();

Of course, you can't access the values of $_SESSION on another page once you call session_destroy, so it doesn't matter that much.

Try the following:

session_destroy();
$_SESSION = array(); // Clears the $_SESSION variable

After using session_destroy(), the session is destroyed behind the scenes. For some reason this doesn't affect the values in $_SESSION, which was already populated for this request, but it will be empty in future requests.

You can manually clear $_SESSION if you so desire ($_SESSION = [];).

Perhaps is way too late to respond but, make sure your session is initialized before destroying it.

session_start() ;
session_destroy() ;

i.e. you cannot destroy a session in logout.php if you initialized your session in index.php. You must start the session in logout.php before destroying it.

I had to also remove session cookies like this:

session_start(); 
$_SESSION = []; 

// If it's desired to kill the session, also 
// delete the session cookie. 
// Note: This will destroy the session, and 
// not just the session data! 
if (ini_get("session.use_cookies")) { 
    $params = session_get_cookie_params(); 
    setcookie(session_name(), '', time() - 42000, 
        $params["path"], $params["domain"], 
        $params["secure"], $params["httponly"] 
    ); 
} 

// Finally, destroy the session. 
session_destroy();

Source: geeksforgeeks.org