If you have a system which uses separated PHP inis for loading extensions (Debian, Fedora and most other distributions do so), [PATH=] or [HOST=] will prevent loading extensions defined in those ini files (extension= and zend_extension=).
I had some headache, when I added a hosts.ini to /etc/php5/cgi/conf.d (Debians default), where I wanted to define some defaults for a host name. The manual says extension= and zend_extension= aren't allowed in [HOST=] and [PATH=] (see above). I figured out that you can easily fix that with adding [PHP] after your definitions.
For example hosts.ini:
[HOST=dev.example.com]
display_errors = on
[PHP]
this will change the section back to [PHP] where extension= and zend_extension= is allowed.
