array storing

iam tring to store an array using wp-sessions.my code shown below.

if(isset($_POST['id']))
		{
		$wp_session = WP_Session::get_instance();
		if(isset($wp_session['ym_cart']))
		  {
		$wp_session['ym_cart'][]=$_POST['id'];
		 }
                  else
		{
		$wp_session['ym_cart']= array();
		$wp_session['ym_cart'][]=$_POST['id'];
		}

		print_r($wp_session['ym_cart']);

	    }

‘ID’ is coming from an ajax form submission.Error is array storing only one value.How can i push values to these array ?

https://wordpress.org/plugins/wp-session-manager/