compute po in php mode

po target now uses php language when extracting keywords.

This allows :
- both `_('…')` and `_("…")` to be used
- inline comments like `// _COMMENT` to be included in generated po

be careful, `#` comments are no more extracted. For workflows or other additional translatable keys, you have to create a separate php file (do not forget to begin it with `<?php`, and give it a `.php` extension)