If your site is infected with some virus like :
then it means your site is infected with eval base64_decode php virus and this code will be in our all of
the server files just after the php tag.
so the remove this code just download the file below and upload it to your server root folder like on many server :
htdocs or may be www and then run this file from our browser like http://xyz.com/eval-remover.php it will remove all the eval base64_decode code from your all the files and your site will be up again.
Please comment as well. 🙂