The best resource out there to learn PHP is the PHP manual. It's extremely well written and well structured, and it's one of the biggest assets of the PHP community. If you are confident with ASP.net, the manual is enough to get you started.
Before you start reading though, you should setup a PHP stack. You can install Apache, PHP and your favorite database (SQLServer is fine) by yourself (it's extremely easy) or you could setup a ready made stack like XAMPP. XAMPP will install:
all in one go (plus some other stuff). One important library it's missing is phpUnit, that obviously will help you with unit testing.
Then you should install an IDE, your obvious choices are Eclipse Classic with the PHP Development Tools (PDT) or Netbeans for PHP. There are a lot of others, but these two I recommend. I use them both, if you can't choose between them Netbeans is a little easier to get the hang of, but you should really evaluate both and decide for yourself which one is better for you - I haven't worked with Visual Studio in a long time and I really don't know which IDE may feel more familiar to you.
When you get over the basics from the manual, you should start thinking of an MVC framework. Again, I will recommend two:
Zend Framework is the beast and CodeIgniter is the beauty, if you allow me a silly metaphor. Zend Framework has a steep learning curve, is backed by Zend (the company behind PHP), and you can do almost everything with it. CodeIgniter is a lot easier to get the hang of, has nice documentation for beginners and it's fairly popular. But most of it's codebase is outdated crap, as they used to support PHP4 not so long ago.
For a beginner the obvious choice is CodeIgniter, but don't get stuck with it. Use it as a learning tool and move to Zend Framework as soon as possible. Again there are a lot more MVC frameworks in the PHP world, but these two are the ones I recommend.
As for books, a great non beginner book is PHP Objects, Patterns and Practice. Sitepoint's PHP books are always a delight to read. If you fall in love with PHP you should really consider a subscription to PHP Architect.
Lastly, check out this great discussion on purely technical reasons for PHP as a first choice.
Update: A few more online resources, that mostly target non beginners:
and if for some weird reason you want to play around with PHP on IIS instead of Apache, a good starting place is Microsoft's PHP on Windows. PHP on IIS works and is a valid choice especially for a .Net developer, but you should really go with Apache (and MySQL or PostgreSQL instead of SQLServer) to get the full PHP experience and also ensure that your skills are portable to any other popular OS. You don't want to miss out on the joy that is mod_rewrite.
Personal note: There's a lot of hate floating around for PHP, some of it is justified. If you do decide to go for it, you must be extremely critical of the various PHP resources, scripts, libraries you will meet along the way, there's too much crap out there and it's impossible for a beginner to know the difference. PHP's vast popularity can be translated to a very big and helpful community, large marketability of your skills, millions upon millions of blogs, but it can also be translated to a few thousand terabytes of crappy code.