You are right Clark, this problem does not show up with a new config.
I don't remember if I did something special to have php files opened in PHP mode (this is what I want anyway as very few files need to be edited in HTML mode for my projects) but I would be interested to find out what can be wrong in my user.cfg.xml file.
What should I look for ?
Attaching it just in case you could have a quick look and spot some obvious setting.
Thanks for your help.