i can`t use cookie helper

[eluser]minhbu[/eluser]
hi !
i cannot use cookie helper but i used se_cookie of php that is well

$this->load->helper('cookie');
$c=array('name'=>'a','value'=>'1');
set_cookie($c);
echo get_cookie('a'); => return empty
setcookie('a2','2');
echo $_COOKIE['a2']; =>return 2