试图创建一个简单的登录系统，非对象错误的属性

Question

Sorry about the probably extremely stupid question but I've been messing with this for hours and still cannot get it to work.

I am trying to create a login system using a mysqli database. For some reason it will not work, i'm currently getting the property of non-object error.

Here is the code

 <?php 

  session_start();

  $db = ("obsidian");
    $mysqli = new mysqli('localhost','admin1', 'password1') or die('Failed to connect to DB' .                          $mysqli->error );

 if(isset($_POST['sub'])){ 

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

}

$sSQL = "select * from users where password='$password' AND username='$username'";

$rsMain = $mysqli->query( $sSQL );

  if ($rsMain->num_rows == 1)

      {

         $_SESSION['userid'] =$username; 
         header("Location: index.php"); 
      }

  else 

     {

         //ignore
     }  

   ?>

It's the checking for the rows that I'm having the issue with, I cannot figure out how to do it.

Any help would be great.

The error is Notice: Trying to get property of non-object in C:\xampp1\htdocs\ObsidianDream\login2.php on line 20