I don't think you can "include" .ini files from the main php.ini file.
One possible solution, though, might be to use this option on the configure line, when compiling PHP :
--with-config-file-scan-dir=PATH Set the path where to scan for configuration files
If this option is used at compile-time, PHP will look for every .ini file in this directory, in addition to the "normal" php.ini file.
I suppose this is what is used by Ubuntu, for instance, which uses a different .ini file for each downloaded extension, instead of modifying php.ini.
The path to the php.ini file being defined with this option, on the configure line :
--with-config-file-path=PATH Set the path in which to look for php.ini [PREFIX/lib]
Still, it probably means you'll have to re-compile PHP -- which is not that hard, btw -- the hardest part being to get the dependencies you need.
And, here is a post on the internals@ mailling-list that says the same thing as I do : config files and