PHP user != open-source enthusiast
When I started developing in PHP way way way back in 2000 I did so because if I didn't, my job was going down the toilet. Basically I'd conned my way into a web design job, realised I couldn't design for shit, sussed that the company had no coders, and swiftly learned to code. Why PHP? Well, I could have had ASP, but I chose PHP and MySQL (the partnership here is important) because there were no cost/licensing issues with either, and security was rock solid. It was also much nicer than that butt-ugly mess known as Coldfusion. My assumptions about security would prove correct, with none of my stuff being hacked, but later my ASP-loving colleagues getting hacked to smithereens.
Anyway, back then, Linux on the desktop wasn't really anyone's idea of fun. I tried a few live cds from magazines god-knows-when (probably 2003-4ish but can't honestly remember) and they were terrible. Linux was for servers, windows was for desktops. It's a habit that stuck til about 2 years ago when I stumbled upon the gateway drug, Ubuntu. These days I'll happily use Arch, BSD, OpenSolaris or any other assorted weirdness for my desktop, because they all mostly let me get my work done (Ubuntu still wins here for 'Just bloody works and stays that way'). I'm now a Linux user.
The thing is, being a PHP user doesn't necessarily make you into open-source. Of course, now I'm a fan, now I know what it is, but back when I started I didn't know jack. I suspect many devs started the same way. Now I develop on Linux because a linux desktop is excellent, but I suspect many may still be in the mindset that Windows is for desktops and linux is for servers. I'm sure they'll eventually move accross, it just takes a couple in each office to stumble upon it then it'll spread like wildfire, provided corporate policy doesn't screw them. Once you have a few early movers, they can show everyone the cube and wobbly windows and all will be right with the world (hey I know they're lame, you know they're lame, but n00bs still coo over them, so there's nothing wrong with eye-candy to attract the masses).