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

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->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">
                 
              
              
        
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