Where is php.ini located by default?

I've built php from a tarball. Now I need to specify a special include path in the php.ini file. But I can't find it. There is a php.in-dist in the build catalog but I can't find a production php.ini anywhere. The docs says that it should be in the install catalogue (which is where?, /usr/local/lib/php doesn't have one) or the cwd. I've tried editing the sample file and putting it in either place but my app still gives me the following error: Failed opening required 'class.DBI' (include_path='.:/usr/local/lib/php') in FILENAME Martin S.