Via this pointer from Adam Trachtenberg today, there's a note on the php cult website about "why you should upgrade to PHP5".

So, I read a post by Chris Shiflett about a book titled Upgrading to PHP 5. I must say, this book is awesome. I’ve only gotten to chapter 2 , but thats because of the excellent writing and explanation of some of the reasons why PHP 5 is doing things the way it is.

I believe I’m in the majority of PHP developers, where we don’t have a formal background in programming. This is a great book to read if you want to figure out what exactly is new in PHP 5, and why you want to use it.

I think if you’re contemplating upgrading to 5, or writing your next app in PHP, you should really read this book and then decide for yourself if you want to do it in 5 or 4.

A good recomendation for an even better book - the "Upgrading to PHP5" from Adam provides information on everything from the new MySQL extension, rewritten support for XML/XSL/DOM, and the enhanced SOAP web service support...