Htacess : rewrite base url without vhost access

I'm renting a multi-domain server. There is few folders on this server like : - www - site1 - site2

This folders are website, associated like :

• www -- www.siteA.fr
• site1 -- www.siteB.fr
• site2 -- www.siteC.fr

My problem is that in site2 Folder, there is a Symphony 2 website configured : site2/web/app.php must be the first url called when user go on www.siteC.fr but I can't modify VHOST Because it's a multi domain server.

So, I'm trying to write a .htacess file with rules below : - When ./site2 is called with www.siteC.fr, display ./site2/web/app.php, always like www.siteC.fr

How can I do that ?