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I have upload my site to my server, but he can't find the folder /includes/header.php. Can I echo the location where he looks to the file. The only error I see now is:
Warning: include(/includes/header.php) [function.include]: failed to open stream.

But what is the "start" location? Can I echo this?

EDIT :

The thing I want to know is where the location is when you use include("/blabblabla").
So the "home path"

$_SERVER['DOCUMENT_ROOT']

The document root directory under which the current script is executing, as defined in the server's configuration file.

include $_SERVER['DOCUMENT_ROOT'].'/includes/header.php';

__DIR__

The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(FILE). This directory name does not have a trailing slash unless it is the root directory.

include (__DIR__"/includes/header.php");

Font: DIR

getcwd() Gets the current working directory

include getcwd()."/includes/header.php";

Font: getcwd()

The php include path will take a file system path as a string parameter. The path can be either absolute e.g.

 include('/home/yourusername/public_html/includes/somethingtoinclude.php');

or relative to the location of your script. So if your script is in /home/yourusername/public_html/ and your includefile is in /home/yourusername/public_html/includes/ the following will work

 include('includes/somethingtoinclude.php');

Note that include will also search in all directories in the php include path as set in php.ini in the include_path directive