Why don't htaccess variables work?

Question

Apache HTTP Server

Hello, I'm trying to make it so that, for example, with a 404 error, when redirecting, the address to which the user tried to go is transmitted.
I do it like this:

ErrorDocument 404 /error_handler.php?error=404&add=%{REQUEST_URI}

But in the end nothing works. Instead of a request, this variable remains.
All htaccess:

Options +FollowSymLinks
RewriteEngine On

RewriteRule ^(\w+)$ $1.php [NC]
Options All -Indexes
DirectoryIndex main.php

ErrorDocument 404 /error_handler.php?error=404&add=${REQUEST_URI}
ErrorDocument 403 /error_handler.php?error=403&add=${REQUEST_URI}
ErrorDocument 400 /error_handler.php?error=400&add=${REQUEST_URI}

Server on CentOS, Apache

Answer 1

error_handler.php if you look what comes to $_SERVER?