The code has been put there through some sort of other vulnerability on the site. Here's the deobfuscated PHP, run at your own peril. It looks like some sort of shell which would allow attackers to run certain commands/farm information on the server it's hosted on
https://gist.github.com/jtylr/4fd6240ddcd046e62535
The code has been encoded and compressed, base64_decode()
decodes the string, gzuncompress()
decompresses it and eval()
(see: evil) will then run the string.