SID constant has no output

Question

<?php
if(isset($_GET['PHPSESSID'])){ 
session_id($_GET['PHPSESSID']);
}
session_start();
echo "SID".SID;
//产生的SID格式为：Session name=Session ID
?>
<br/>
<a href="session.php?<?php echo SID?>">刷新</a>