PHP MySQL选择问题

为什么我的查询没有返回任何结果?我可以运行查询SELECT * FROM用户WHERE wid = 'worker_040'，它返回的结果很好。我遗漏了什么吗?

$wid = $_POST['username'];
            $con = mysql_connect("11.88.3.2","XXXX","XXXXX");
            if (!$con)
                {
                    die('Could not connect: ' . mysql_error());
                }
            mysql_select_db("chit", $con);
            echo "Form details<br />";
            $result = mysql_query("SELECT * FROM `users` WHERE wid = '$wid'");
            while($row = mysql_fetch_array($result)){
            echo $row['FirstName']. " - ". $row['LastName'];
            echo "<br />";
            }
        mysql_close($con);