Unable to return session value in views Field
You need to remove session_start() from Rules. Drupal calls session_start() when firing drupal_session_start().

Check out session_start manual on php.net


  As of PHP 4.3.3, calling session_start() after the session was
  previously started will result in an error of level E_NOTICE. Also,
  the second session start will simply be ignored. 


Screenshot for PHP field settings in Views: