You need to make sure everything is processed in the correct order.

For example, I just used vanilla apache/php from osx 10.6.4:

    $ cat /etc/apache2/httpd.conf
    ...
    LoadModule php5_module        libexec/apache2/libphp5.so
    ...
    Include /private/etc/apache2/other/*.conf

    $ cat /etc/apache2/other/php5.conf 
    <IfModule php5_module>
            AddType application/x-httpd-php .php
            AddType application/x-httpd-php-source .phps

            <IfModule dir_module>
                    DirectoryIndex index.html index.php
            </IfModule>
    </IfModule>