为什么不是'；t我的变量是用SESSION变量设置的

我认为这是一个语法问题，但我已经尝试了几种不同的方法。在IIS 6上运行的PHP 5.4.16（这些不是我的选择）。

我无法将$usr设置为$_SESSION['uid']。我在设置后立即运行了一个转储，我看到了会话数据的uid信息，但$usr为NULL。语法错误？你认为发生了什么事？

function User_CustomValidate(&$usr, &$pwd) {
    session_start(); // Initialize Session data
    ob_start(); // Turn on output buffering
    $appKey = "pwssssssssssssss";
    $safeurl =  'https://safe.ssssss.com/login/sso/SSOService?app=playbooks';
    // first call back after safe login - POST is set
    if ($_POST && isset($_POST['digest'])) 
    {
        $digest = $_POST["digest"];
        // set the session variables ...
        $_SESSION['usernames'] = $_POST["firstname"]." ".$_POST["lastname"];
        $_SESSION['firstname'] = $_POST["firstname"];
        $_SESSION['lastname'] = $_POST["lastname"];
        $_SESSION['email'] = $_POST["email"];
        $_SESSION['uid'] = $_POST["uid"];
        // Needed for key
        $uid = $_POST["uid"];
        $time = $_POST["time"];
        // Read the property file with the key and URL so this won't go into the main code ...
        // this sets $appKey and $safeurl
        $mykey = "".$uid.$time.$appKey;
        $mydigest = md5($mykey);
    }
    // session is not initialized as we never got the post above to set session vars
    // call now the safe login to get the post to set the session vars ...
    if (!isset($_SESSION['uid']) || empty($_SESSION['uid']))
    {
        // Read the property file with the key and URL so this won't go into the main code ...
        // this sets $appKey and $safeurl
        header("Location: ".$safeurl);
    }   
    $usr = $_SESSION['uid'];  
    var_dump($usr, $_SESSION['uid']);                    
    $this->setCurrentUserName($usr);
    return TRUE;                    
}     

因此var_dump显示$usr=NULL和$_SESSION['uid']，其中SSO传递了正确的员工ID。