各位高手們好:
最近剛開始學程式，在試著從資料庫抓資料寫入SESSION時，出現了我不知該如何檢測排除的錯誤:
即資料庫有找到資料時，程式可正確顯示"使用者EMAIL，歡迎光臨。"
但，資料庫沒找到資料時，畫面卻只顯示一片空白(本預期該顯示帳密錯誤)。
不知道有沒有人可以給我一點提示怎樣查問題 ?
無論結果如何，都十分感激 ~

謝謝

<?php  
session_start();
include '../common/config.php';
include '../common/define.php';
include '../common/utility.php';

$ss_role_id = isset($_POST["u_role_id"]) ? $_POST["u_role_id"] : "";  
$ss_u_email = isset($_POST["u_email"]) ? $_POST["u_email"] : "";
$ss_u_password = isset($_POST["u_password"]) ? $_POST["u_password"] : "";

$pdo = db_open();

// 寫 SQL 語法
$sqlstr = "SELECT u_email, u_password, u_role_id, uid, u_fname FROM users WHERE u_email=? and u_password=? and u_role_id='2' ";
$sth = $pdo->prepare($sqlstr);
$sth->bindValue(1, $ss_u_email, PDO::PARAM_STR);
$sth->bindValue(2, $ss_u_password, PDO::PARAM_STR);

if($sth->execute())
{
   // 成功執行 query 指令
   //$total_rec = $sth->rowCount();
   $ss = '';
   $msg ='';
   $html = '';
   $ss_uid ='';
   while($row = $sth->fetch(PDO::FETCH_ASSOC))
   {
      $ssdb_u_email = convert_to_html($row['u_email']);
      $ssdb_u_password = convert_to_html($row['u_password']);
      $ssdb_u_role_id = convert_to_html($row['u_role_id']);
      $ssdb_uid = convert_to_html($row['uid']);
      $ssdb_u_fname = convert_to_html($row['u_fname']);

	   // echo $ssdb_u_email;
	   // echo $ssdb_u_password;
	   // echo $ssdb_uid;
	   // echo $ssdb_u_fname;	   
	   // exit;

	    $valid = false; 		
	  	if($ssdb_u_email==$ss_u_email && $ssdb_u_password==$ss_u_password && $ssdb_u_role_id=='2')
		{

		   $valid = true;
		   $_SESSION["u_role_id"] = $ssdb_u_role_id;
		   $_SESSION["u_email"] = $ssdb_u_email;
		   $_SESSION["u_password"] = $ssdb_u_password;
		   $_SESSION["uid"] = $ssdb_uid;
		   $_SESSION["u_fname"] = $ssdb_u_fname;
		   //$msg = $ss_u_email . ' 帳密正確你好，歡迎光臨！ '; 
		   echo $ss_u_email . ' 帳密正確你好，歡迎光臨！ ';
		   exit;
		}
		else
		{  
	 	//這區判斷秀不出來!!!
		echo 'x'; 
		   // $_SESSION["u_role_id"] = '';
		   // $_SESSION["u_email"] = '';
		   // $_SESSION["u_password"] = '';
		   // $_SESSION["uid"] = '';
		   // $_SESSION["u_fname"] = '';
		   //$msg = '登入錯誤';
		echo  ' 帳密錯誤！ ';
		exit;
		}
    }	

}
else
{
   // 無法執行 query 指令時
   $html = error_message('list_all');
   echo $sqlstr;
}

include '../web/pagemake.php';
pagemake($html, ''); 
?>