According to this article in The Register, more PHP developers (of the ones polled by Zend) prefer to do their actual development on a Windows platform, but still host their sites on something like Linux (as shown in the results of a Zend survey).

In a recent study from Zend Technologies, forty two per cent of PHP programmers named Windows as their primary development operating system. Linux came second, with 38.5 per cent, while Mac's OS X was third on 19.1 per cent. Zend did not say how many developers it spoke to, but called the December poll a "global survey" ranging from independent consultants to organizations with more than 5,000 employees.

The same survey shows that the choice of Linux for the server is still strong, and that the rest of the options (Windows, OS X) are trailing by a large percentage. You can read Zend's official press release on the survey on PRWeb.net.