我不知道为什么它不会返回结果

Question

我不知道为什么它不会返回结果，虽然我正确登录，但是在运行程序时它仍然是假的

 <?php
    $u = "";
    $p = "";
    if(isset($_POST['ok'])){
        if($_POST['txtuser']==NULL){
            echo "Please Enter Your Username<br/>";
        }else{
            $u = $_POST['txtuser'];
        }
        if($_POST['txtpass']==NULL){
            echo "Please Enter Your Password<br/>";
        }else{
            $p = $_POST['txtpass'];
        }
        if($u && $p){
            $conn = mysqli_connect("localhost","root", "", "phponline") or die ('connect fail');
            $sql = "select * from user where username='$u' and password='$p'";
            $query = mysqli_query($conn,$sql);
            if(mysqli_num_rows($query) == 0 ){
                echo "Wrong username or password, please try again";
            }else{
                echo "Login Success";
            }
        }
    }

    ?>