不能修改常量的值？该怎么解决

if(empty(SESS_ID))
{
 SESS_ID = 'xxwe';
}

define('SESS_ID', $sess->get_session_id());

#==============================

if ( !defined('SESS_ID') ) {
    echo "木有定义哦~";
    define('SESS_ID', 'xxwe');
}