several conditions in apache mod rewrite to make custom URLs - php

Question

In my domain example.com , I want to put all page files in a directory at root for example named file. So for example I put about.php the file directory and want to let users to access this page with this URL : example.com/about
or for example put user directory in the file directory and put login.php in it , and want to let users to access it with : example.com/user/login
In fact I want to remove .php and the file from the URLs.
AND
If those files don't exist , it should load a default file like index.php at root. for example the URL example.com/blabla should be mapped to the index.php
In fact , I want to make two conditions in mod rewrite with the mentioned priority.
notice : of course I should be able to use variables like $_GET at files like about.php
UPDATE : in summary , it should work with this logic :
if the URI isn't a file or directory{
if file/URI.php is a file
load file/URI.php
else
load index.php
}
some body gave an answer to use ErrorDocument 404 index.php , but it's a really bad idea (and now has deleted his answer !)
Thanks for your help...

Answer 1

A conditional rewrite (if the pages exist) sounds like a bad and complicated idea, if it even is possible.
I would rewrite everything to /index.php and control everything from there:
If the components of the url lead to an existing page, use / include that content;
If no valid page is found, present your original index content.
Example redirect rules for your situation:
RewriteEngine on
RewriteBase /
RewriteCond %{SCRIPT_FILENAME} !-f
RewriteCond %{SCRIPT_FILENAME} !-d
RewriteRule ^.*$ /index.php?id=$1 [QSA,L]
Will include the requested path in id and append the variables from the original query string (the QSA flag).
More specific for your /user/login example would be:
RewriteRule ^([\w-]+)/([\w-]+)$ /index.php?path=$1&file=$2 [QSA,L]
[\w-+] : any word character (including _) and the -