Philthy,
I work in both environments so i know the culture shock that can ensue from changing.
I've worked both sides of the coin for so long now that working in both languages is second nature.
If your doing C#/ASP.NET then you'll likley have an IIS Webserver installation. The good thing is, that you can just head to the PHP web site and download the windows binary. Running that and answering the questions appropriate for IIS will install the PHP engine under IIS and you can then use the visual studio IDE to develop PHP scripts.
You won't however get syntax & colour highlighting or anything like that, but you will get the advanced editing features that the VS editor has available.
If you want colour highlighting, then i use "ultra edit" for all my editing, and run my scripts from the command prompt using "PHP -q <scriptname>"
If your running under linux, then depending on your install you can use RPM or APT-GET to install the required parts.
Watch out for my new series of articles called "The ABC of PHP" appearing on PHPBuilder soon, that should give you a good starting point.
As for Pear i really don't use any of them, but your milage may vary. My advice is to not use any toolkits or libraries until your comfortable with the language itself, trying to get to grips with high level libraries & toolkits before you understand the language will very likley confuse you.
Cheers
Shawty