When I started answering this question, I realized that this probably will be a multi part post.
So let me start with a little bit of history.
Prior to 5.3, PHP on Windows (although it existed, and was heavily used for development) was a little bit a bastard child of all the available PHP releases on any platform.
Building PHP for example on Windows was not for the faint of hearted. It was build using Visual Studio 6 (So, what does that mean? Well that is virtual studio 1998) Which makes that a 10 year old compiler.
To make matters even more interesting, many of the libraries used in PHP 5.2 and before where last build in 1998 and 1997. And the source code for those libraries had been pretty much lost to the ages.
So debugging PHP on Windows was virtually impossible to do. And fixing bugs, security issues and the like that for those libraries was not really possible.
Additionally, only a handful of people had any idea (or sickness of mind

) how to build PHP that you could download from php.net. (Special thanks for example go out to Elizabeth Smith, one slightly insane PHP soul who build it for many years in the community

)
It is one of the reasons that the PHP community late last year stopped building/supporting PECL for Windows, because it was such a pain to build PHP for those people who wanted to build extensions! (Something that is about to be remedied btw

)
Than of course there is this issue of performance, PHP on Windows has never been as fast in the past as it has been on Linux.
So taken all of that, it was quite a monumental task to get PHP on Windows to be a first class language. Something that Microsoft could never do without working very closely with the PHP community.
But that is what we did, after much meetings with lawyers at Microsoft (I think I met 99% of them now) we started working on improving PHP on Windows. And that work started about 10 months ago.
But more about that in the next post...................