I searched for git PHP implementations too, but with no results. The only way to re-create a site similar to GitHub is to setup a "real" git server on your own server and then use a PHP git web client like http://www.xiphux.com/programming/gitphp/.
Unfortunatly, you can forget to do this with a hosting solution. You need a real virtual server where you can install everything you want.
However, if you need a place where store some personal, non-public, non-accessible-to-everyone code for a project, you can try BitBucket. It offers private and private-shared git repos for free.