Many thanks BenPleyser,
Until now, Chrome was able to display php pages without having any web servers installed, simply by pressing F12 in DW or even opening a document in any folder.
Or at least, it's what I think I was doing. Maybe that somebody has an explaination.
Chrome without a testing server cannot render anything beyond ordinary HTML, CSS and JS code. Either you had a testing server installed and didn't know it or your page didn't contain any server side scripts.
When working with PHP code, DW always asks me to set-up a testing server if one is not already defined.
Screenshot without a testing server.


With local testing server.
