I'm using Wordpress with the SEO frienly URL's. This works fine.

Now I want to integrate a "non-wordpress" page. I've made a page link mydomain.com/custom.php. Every person who is trying to get to http://www.mydomain.com/custom/ or (www.mydomain.com/custom/custom2/etc) should go to http://www.mydomain.com/custom.php.

That's the only change, offcourse the folder /custom/ is not used anywhere in the wordpress URL structure.

The current .htaccess is:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

What i tried is:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^/custom/([^/\.]+)/([^/\.]+)/?$ /custom.php?cat=$1&subcat=$2 [L]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Unfortunately my wordpress theme is still saying: OOPS, THE PAGE YOU ARE LOOKING FOR CANNOT BE LOCATED.

Anyone knows what the problem is here?