PHP - Call to a member function query() on a non-object

<?php

if(!empty($_POST)){

	if(isset($_POST["username"]) &&isset($_POST["password"])){

		if($_POST["username"]!=""&&$_POST["password"]!=""){

			include "conexion.php";

			$user_id=null;

			$mysqli= "select * from user where (username=\"$_POST[username]\" or email=\"$_POST[username]\") and password=\"$_POST[password]\" ";

			$res = $mysqli->query($sql) or trigger_error($mysqli->error."[$sql]");

			while ($r=$query->fetch_array()) {

				$user_id=$r["id"];

				break;

			}

			if($user_id==null){

				print "<script>alert(\"Acceso invalido.\");window.location='../login.php';</script>";

			}else{

				session_start();

				$_SESSION["user_id"]=$user_id;

				print "<script>window.location='../home.php';</script>";

			}

		}

	}

}

?>