Detect if PHP session exists

I use a combined version:

if(session_id() == '' || !isset($_SESSION) || session_status() === PHP_SESSION_NONE) {
    // session isn't started
    session_start();
}

If $_SESSION (or $HTTP_SESSION_VARS for PHP 4.0.6 or less) is used, use isset() to check a variable is registered in $_SESSION.

isset($_SESSION['varname'])

If you are on php 5.4+, it is cleaner to use session_status():

if (session_status() == PHP_SESSION_ACTIVE) {
  echo 'Session is active';
}

• PHP_SESSION_DISABLED if sessions are disabled.
• PHP_SESSION_NONE if sessions are enabled, but none exists.
• PHP_SESSION_ACTIVE if sessions are enabled, and one exists.