RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^.]+)$ $1.php [NC,L]

ErrorDocument 400 /errors/400.php
ErrorDocument 401 /errors/401.php
ErrorDocument 403 /errors/403.php
ErrorDocument 404 /errors/404.php
ErrorDocument 500 /errors/500.php