$username = stripslashes($_POST['username']);
$password = stripslashes($_POST['password']);

$query1 = "SELECT * FROM user where username = '$username'";
$result1 = mysqli_query($conn, $query1);

$query2 = "SELECT * FROM user WHERE password = '$password'";
$result2 = mysqli_query($conn, $query2);

if(!($result || $result2)) {
        echo "query failed";
}
if(mysqli_num_rows($result1 && $result2) <= 2) {
        header ("Location: mainpage.html");
} else {
        echo "wrong username or password";
}