问如何解决 mysqli_num_rows() expects parameter 1...line？

代码如下

<?php

include_once("php/common.php");

$userName = $_POST['userName'];

// $password = $_POST['password'];

$userName = addslashes($_POST['userName']);

$password = addslashes($_POST['password']);

$loginSQL = "select * from users where userName='$userName' and password='$password'";

$resultLogin = mysqli_query($loginSQL);

if (mysqli_num_rows($resultLogin) > 0) {

echo "登录成功";

header("Location:../manage.php");

} else {

echo "登录失败";

}

?>