eval() functions are often used by hackers to add encoded code into old/vulnerable files getting it to do various things. This was likely injected code by a hacker. As suggested by quanta, run a grep in your directory to find all occurrences of this. If need be, review your files one at a time. Someone may have gained access to your server through vulnerable code.
The 'split()' function may also be encoded so searching for "split" may come up empty. In this case, view the following file in a text editor (I'd recommend not running it in a browser at all):
Most code is either injected right at the top or on the very bottom. It's usually encoded into base64. Look for gibberish text at either of these locations. If it doesn't exist in either of these places, start reviewing the code line-by-line.
Also, a lot of hackers will change the permissions of hacked files to read, write, execute for all users (777). Have a quick view of the permissions column via FTP to see if other files may have been altered.