I had a positive experience using gcov with a C extension. I used it to get a better sense of what the .phpt tests (and valgrind) were really telling me.
To get it to work in an automated way, I hacked out Makefile.gcov and some relevant chunks of config.m4 from php-src. I'm wondering if there would be value in getting phpize to produce some of this. Am I right in guessing that other people want to do things like this? Thanks, Adam