How to check for data on a popen()ed FD?

I'm running a program using popen(), and I need to know when there's data to be read from this program. feof() will never return true, even with no data to be read. Is there any function that can set a timeout on a file descriptor returned by popen()? Or a function like feof(), but to check if there's data to be read from a popen() file descriptor? What I'm doing here is running Crafty (a chess engine), from my PHP script, and I need to get input from the user on stdin and from Crafty. Any help is appreciated! Thanks!