In a new blog entry, Christopher Kunz talks about an issue he had when upgrading his MySQL version to 5.0.22 and recompiling with PHP.

I ended up with the following error at make install: PHP Fatal error: Unable to start curl module in Unknown on line 0

Finding that it was a MySQL issue, he set about trying to find the issue (turned out to be a badly linked SSL library on MySQL's part). This issue is only in the MySQL binaries, though, so if you come across it you'll just need to compile MySQL by hand first, then compile PHP against it.