This hack is all over the internet... the .htaccess file redirects whenever someone tries to go to a page on your site that doesn't exist to the php file. Then the php file sends them to their own website. The hackers are posting links all over the internet as we speak to pages that don't exist on your site.. so when someone clicks it they will be redirected to their own site. Here is my blog from when it happened to me months ago. You need to delete those files QUICK and hope Google doesn't penalize see more you.

http://www.boonex.com/unity/blog/entry/EVERYONE_PLEASE_READ_