不能修改常量的值？该怎么解决

不能修改常量的值？
define('SESS_ID', $sess->get_session_id());

PHP code

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

为什么报错啊

------解决方案--------------------

PHP code

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

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

if ( !defined('SESS_ID') ) {
    echo "木有定义哦~";
    define('SESS_ID', 'xxwe');
} <div class="clear">
                 
              
              
        
            </div>