Why does PHP if-stament return true when it was planned not to [closed]

Code:

if($_SERVER["REQUEST_METHOD"] == "POST"){
    $queryCommand = "SELECT * FROM `users` WHERE `username` = '$username' AND `password` = '$password'";
    $res1 = mysqli_query($connection, queryCommand);
    if ($res1 == FALSE && (empty(trim($_POST["username"])) || empty(trim($_POST["password"])))){
        $errorTxt = "Username or Password is incorrect!";
    } else {
        if ($res1 == FALSE) {
            $errorTxt = "Username or Password is incorrect!";
        } elseif ($res1 != FALSE) {
            session_start();

            $_SESSION["loggedin"] = true;
            $_SESSION["id"] = $id;
            $_SESSION["username"] = $username;  

            header("location: index.php");
        }
    }
}

When I refresh the page, it somewhy writes "Username or Password is incorrect!" to the HTML paragraph ( <h4 id="logErrText"><?php echo $errorTxt;?></h4> ). Help would be appreciated.