Get the site root host name dynamically in .htaccess - php

Question

I am trying to make 404, 500 etc pages in ErrorDocument in php .htaccess file, it works fine if give
ErrorDocument 404 http://localhost/project/errordocs/404.html
but i do not want to hard code the url here, instead i want to get the root url site name dynamically so that i do not have change it again and again as the hostname changes.
Basically i want to get this root url like: http://localhost/project can change to http://www.example1.com/project or http://www.example2.com/project etc. This url must come from projects root folder.
So that will dynamically become:
ErrorDocument 404 http://localhost/project/errordocs/404.html
ErrorDocument 404 http://www.example1.com/project/errordocs/404.html
ErrorDocument 404 http://www.example2.com/project/errordocs/404.html
Any help please?

Answer 1

The asker asks incorrect. All what he writes
ErrorDocument 404 http://localhost/project/errordocs/404.html
ErrorDocument 404 http://www.example1.com/project/errordocs/404.html
ErrorDocument 404 http://www.example2.com/project/errordocs/404.html
may be done by
ErrorDocument 404 /project/errordocs/404.html
But really he want: while moving site from project folder to project1, he should not change the rule
I think it may be done by htacces placed in /project with code
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ errordocs/404.html [L]
It will work if AllowOverride set to All (it is default).
The problem only that responce in this case will be 200 but not 404

Answer 2

Based on the conversation you had, you would like to not have to change the path to the error document when you move your project to another folder with a different name.
Firstly, you cannot use variables when using ErrorDocument. The path that you provide must be static. The path that you specify must be either to an external URL (in which case your browser will be redirected) or to a file relative to your document root (i.e. localhost).
Unfortunately, ErrorDocument won't be able to find the file relative to the current directory (i.e. project). The only logical way of doing this would be to remove the leading slash, but this would cause Apache to render that as a string in the browser.
This brings us to the only other possible solution: mod_rewrite. The only problem with using it, however, is that it is processed early on in the mapping pipeline which may allow other modules (such as mod_proxy) to affect the process.
That said, you can try with the following:
/project/.htaccess:
RewriteEngine on
# Determine if the request does not match an existing file or directory
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# If so, send the request to the applicable file, relative to this directory
RewriteRule ^ errordocs/404.php [L]
# Per your comment and suggested edit, add the following.
# Note: This should not make any difference as mod_rewrite and PHP should
# already handle the error document.
ErrorDocument 404 /errordocs/404.php
/project/errordocs/404.php:
This file will send the 404 header as .htaccess won't be able to do that.
<?php header("HTTP/1.0 404 Not Found"); ?>
<h1>Sorry, we couldn't find that...</h1>
<p>The thing you've requested doesn't exist here. Perhaps it flew away?</p>