Thank you very much !
Has somebody already done this kind of job before ? Because I'm not too keen on reinventing the wheel.
The easiest thing I found for now is installing the extension PHPModern, and switching Syntax Definition from PHP to PHP5 when I want to highlight PHP only. But this is not optimal because the PHP5 Syntax Definition colors only PHP, where it is the HTML.tmLanguage file that is used for PHP files by default (as seen in the PHP.tmLanguage).