My website is being redirected to a non-existent page (/showpage.php) for some users when they hit the homepage.  For other users, the site works fine. I have looked in the index.php files and some other commonly used files.  I believe strongly that this is a hack but I don't know how to get rid of it.Any advice on where to start looking? Any experience dealing with something like this? I would greatly appreciate even the slightest thought on the matter.
Website hack resulting in auto redirect - where to start looking?
php;apache;security;hacking