How to use session in wordpress in plugin development -

i new write plugin ..i having testplugin.php file , ajax.php file ..

my code in testplugin.php

global $session;  print_r($abc); //$abc array of data ..  $session['arrayimg']=$abc; //saving data in session   echo  $session['arrayimg']; //displayin "array" 

and ajax.php consists of following code

global $session;  $abc = $session['arrayimg'];  print_r ("abs== ".$abc); //displayin "abs== array" 

and if use session_start();

i following error

warning: session_start() [function.session-start]: cannot send session cache limiter - headers sent  

i want send array of data 1 file of plugin file ...

// on plugin or themes functions.php

function register_session(){     if( !session_id() )         session_start(); } add_action('init','register_session'); 

// set session data -

$_session['arrayimg'] = $abc; 

// data on ajax hooked function -

function resolve_the_ajax_request(){     if( !session_id())         session_start();      $abc = $_session['arrayimg']; }