Document Root fix in .htaccess when using VirtualDocumentRoot

CMS often come with a .htaccess that has a RewriteRule like this:

# RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ app.php [QSA,L]

The RewriteBase has to be enabled if you are using a VirtualDocumentRoot but when you are sharing code, developers may all have different setups.

By checking the content of DOCUMENT_ROOT, we can guess which setup we are using and prepend a / when necessary.

Note however that this method is heavy on string comparison which is slow and should not go on production.