The last paragraph of your question is misleading, at least for me. If your goal is to find a way to invoke service as user Phing without getting asked for a passphrase I would do it with sudo. I will try to answer you question as I understand it, since I don't grasp what speak against the use of sudo in your case.
Just add the line
Defaults exempt_group=Phing
to /etc/sudoers. This line adds the user Phing to a group on which sudo doesn't lay down path and password requirements.
Further you have to add the line
Phing ALL=(root) /full/path/to/service
or alter an existing line of Phing, so that sudo grants him access to service.
After that
sudo service php5-fpm restart
shouldn't ask Phing for a passphrase any longer.
If you want it a bit more strict you can instead add the line
Phing ALL=(root) NOPASSWD: /full/path/to/service
without using exempt_group. Then you have to give sudo the full path of service (if it's not located in the standard paths), so command spoofing isn't possible.
If you want to save some typing, you can also alias sudo service or sudo /full/path/to/service in your shell with service.