While not specifically PHP-related, almost all of the coders that I know out there are effected by this one way or another (I know I am) - comment spam on their blogs. Harry Fuecks has even written up a new posting on just that topic and how bothersome it has really become.

The subject of blog comment spam bothers me, not so much as a problem in itself but because there's alot of people talking about it (and suffering from it) while, at the same time, little real technical analysis.

[...] First have yet to fully answer for myself whether the problem is primarily human beings, manually posting spam, or automated processes (scripts)? assume the answer is both but the bigger problem is the latter, given the volumes being generated in some places, to the point of denial of service against Movable Type.

He continues on, talking about some suggestions to try to wrangle this problem, including a blacklist service, making it more difficult for spam script developers (with the suggestion of making the comment API unique for each install of a system), or even just uniquely identify the form fields for your comment box.

Be sure to check out the comments for more ideas and feedback on Harry's suggestions.