localhost vs. remote server htaccess values - php

Question

I have set up Usbserver as my local webserver on my PC so that the server root is http://localhost:8081/ but all my webprojects are in http://localhost:8081/WWW/PHP/ subdirectory of my local server.
Now: all is working fine - all relative paths work as they should but as I play with the .htaccess file and I want to set up my custom 404.php page with relative path like this:
ErrorDocument 404 /php/errors/404.php
...it just do not work as expected cos it, of course, points to http://localhost:8081/ instead of, let's say, http://localhost:8081/WWW/PHP/myproject/ that is my actually acting kind-of server root (cos once uploaded to remote server it would become http://myproject.com/).
This would be absolutely fine on remote server but not here on my local server structure. I also try one suggestion like this:
ErrorDocument 404 ./php/errors/404.php
...but it ended up with blank white page with sentence "./php/errors/404.php" displayed.
QUESTION: how to write the code in .htaccess "universally", so it would think that root server is in "myproject" directory instead of "http://localhost:8081/" but once uploaded on my remote server, let's say http://myproject.com/ it would still work as normally?
REMEMBER: I cannot use rewriting the root of my local server in Usbserver settings cos I have also quantum of other webpages in "http://localhost:8081/WWW/PHP" so they all would stop working that way!
I guess there is some kind of RegEx code that would solve this (as I saw many times here on Stackoverflow guys do this kind of stuff that way) but I am quite bad in RegEx myself so without help from others I am "lost".
I searched web already before asking this here: there were many solutions but I did not find any that would actually solve this specific task (most of them suggesting just rewriting server root in my Virtual Server - Usbserver - settings which I cannot do for reason explained above).
I simply need it to work OK on localhost and on remote server afterwards without rewriting my .htaccess code everytime I upload my website to remote server and vice versa.
P.S.: I also noticed that when I set up my custom 404.php page with absolutle path for my local server like:
ErrorDocument 404 http://localhost:8081/WWW/_PHP_/myproject/php/errors/404.php
...it works BUT it change URL to that custom 404 page instead of staying at the original "not existent" URL address - why? Cos with normal not-edited 404 page the URL doe snot change, it stays the same (= that non existent link).
EDIT - FINAL SOLUTION:
So after hours of playing, browsing web, searching for any bits of some additional useful information I finally got it working exactly as I wanted - different directory for error pages (404 etc.) specified in case of localhost and remote server, here it is (mind you: it is my own code so it may not look good - I am newbie in .htaccess stuff - but at least it really work for me as I expected it to work):
# RULES FOR ERROR PAGES DIRECTORY IN CASE OF localhost:8081
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} ^localhost:8081 [NC]
RewriteRule .? /WWW/_PHP_/myproject/php/errors/404.php [L]
# RULES FOR ERROR PAGES DIRECTORY IN CASE OF remote server (actual webpage on public server)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} !^localhost:8081 [NC]
RewriteRule .? /php/errors/404.php [L]

Answer 1

Relative path is not allowed in error document directive. so you can use an absolute path instead of the full url.
ErrorDocument 404 /www/_php_/myproject/php/errors/404.php
Reference :
https://httpd.apache.org/docs/2.4/custom-error.html

Answer 2

This is the working solution
# RULES FOR ERROR PAGES DIRECTORY IN CASE OF localhost:8081
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} ^localhost:8081 [NC]
RewriteRule .? /WWW/_PHP_/myproject/php/errors/404.php [L]
# RULES FOR ERROR PAGES DIRECTORY IN CASE OF remote server (actual webpage on public server)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} !^localhost:8081 [NC]
RewriteRule .? /php/errors/404.php [L]