I have recently bought a new MacBook Pro. Before I had my MacBook Pro I was working on a website on my desktop computer. And now I want to transfer this code to my new MacBook Pro.

The problem is that when I transfered the code (I put it on Dropbox and simply downloaded it on my MacBook Pro) I started to see lots of error messages in my PHP code.

The error message I”m receiving is:

> 
> Warning: Cannot modify header information - headers already sent by (output started at /some/file.php:1) in /some/file.php on line 23
> 
> 
> 

I have done some research on this and it seems that this error is most frequently caused by a new line, simple whitespace or any output before the `<?php` sign. I have looked through all the places where I have cookies that are being sent in the HTTP request and also where I'm using the `header()` function. I haven’t detected any output or whitespace that possibly could interfere and cause this problem.

Noteworthy is that the error always says that the output is started at line 1. Which got me thinking if there is some kind of coding differences in the way that the Mac OS X and Windows operating systems handle new lines or white spaces? Or could the Dropbox transfer messed something up?

The code on one of the sites(login.php) which produces the error:

```
 <?php
    include "mysql_database.php";

    login();

    $id = $_SESSION['Loggedin'];
    setcookie("login", $id, (time()+60*60*24*30));
    header('Location: ' . $_SERVER['HTTP_REFERER']);
?>

```

login function:

```
function login() {

    $connection = connecttodatabase();
    $pass = "";
    $user = "";
    $query = "";

    if (isset($_POST['user']) && $_POST['user'] != null) {

        $user = $_POST['user'];

        if (isset($_POST['pass']) && $_POST['pass'] != null) {
            $pass = md5($_POST['pass']);

            $query = "SELECT ID FROM Anvandare WHERE Nickname='$user' AND Password ='$pass'";
        }

    }

    if ($query != "") {
        $id = $connection->query($query);
        $id = mysqli_fetch_assoc($id);
        $id = $id['ID'];

        $_SESSION['Loggedin'] = $id;

    }

    closeconnection($connection);

}

```

Complete error:

```
Warning: Cannot modify header information - headers already sent by (output started at /Users/name/GitHub/website/login.php:1) in /Users/namn/GitHub/website/login.php on line 9

```