Define web root dir - consant works on include but not redirect

Question

I'm trying to create a constant which defines my web root dir, which I can use for constructing paths for requires, header redirects, images, etc.

I need the constant to work when used in files in the root folder and sub folders. I know I could easily hard code the paths with ../ etc, but the main reason I'm trying to get this to work (apart from clean code) is so I can re-use the code on various sites on different servers, where the path may vary.

my dir structure

/
  index.php
    /library
      libSetPathRoot.php

Document Root

echo $_SERVER["DOCUMENT_ROOT"];
Returns: /home/james/web/sites/
The "sites" dir contains various document roots defined in /etc/apache2/sites-available (so I can work on different scripts).

File contents

/index.php
require_once('library/libSetPathRoot.php'); //works fine.. of course

/library/libSetPathRoot.php
define("PATHROOT", realpath(__DIR__ .'/..').'/');

The above returns my root correctly:
echo PATHROOT;// /home/james/web/sites/site3/

I've tried the following and various other code to define a root path.
These, and others, didn't work for redirect(), include(), nothing at all:
define ('PATHROOT', getenv("DOCUMENT_ROOT"));
define ('PATHROOT', basename(dirname((__DIR__))));
echo PATHROOT;
Everything else seemed to return "site3" only.

The working define works for include() and require() regardless of where in the dir structure it is placed. ie if in file in sub dir to root calling root or other sub dir to root, in a file in root, included file etc.

But I just cannot get this working for header() redirect. Again, regardless of where I use it - root scripts, sub folder scripts, included scripts. Maybe I need another approach, but have read numerous options and tried them extensively (half a day spent).

Why is my define working for includes but not header redirect, and how can I make it work on all? Or do I have to use two methods, one for include and one for redirects?

Edit
Example header redirect I tried:
header('Location: '.PATHROOT.'page-not-found.php');
Returns a 404 for: /home/james/web/sites/site3/page-not-found.php
(the file does exist)

Answer 1

You can't do such file redirection with header(). It sends HTTP headers to the browser which should be in browser accepted format. But you are sending some local(server) file path. Which is obviously not working. you have to send a url for redirection eg. header("Location: http://example.com/page-not-found.php")

See the php documentation http://php.net/manual/en/function.header.php

Answer 2

You can use the header() function to send a new HTTP header, but this must be sent to the browser before any HTML or text (so before the declaration, for example).

header('Location: http://www.example.com/');

The URL must be an absolute. See RFC 2616. But in most cases a relative URL will be accepted too.

However header function and include function expect completely different parameters! The header function expect URL and include function expect file-system file object.

Eventually you can tune the header function using relative FS path definitions like "../../newHeader.php"