// check if php session has started
// 1. first and best method 
if(session_id() == '') session_start(); 
// if session is not started, session id will be empty. 

// 2. second method 
if(!isset($_SESSION)) session_start(); 
// if session is not set it should be started. 

// 3. Third method 
if( !defined( 'SID' ) ) session_start(); 
// start session if session id is not defined.