Dreamweaver's support of PHP comes in two forms:
Code editor - not bad.
Point & Click PHP code generation called 'Server Behaviors' - not too good
The code editor, with regards to using PHP, is OK. It color codes, has basic code completion and it has code snippets loaded into buttons (code block creation, session etc ...).
The Server Behavior tools are designed to allow non-programmers to create basic PHP functionality: reading and writing to databases, set session, password protect pages with PHP etc... The problem is, that the code it generates is fragile ... easily broken and in the end, it really is just better to use it for rapid prototyping of simple PHP applications.
... I was a beta tester of Dreamweaver CS4 and I can tell you that even the Adobe developers are aware of the problem; I believe the idea was to do a complete rewrite of of the Server Behaviors for Dreamweaver CS5. In fact, I was writing a book for a major publisher last year on Dreamweaver CS4 and PHP (a lot on Server Behaviors), and I dropped it (after completing 4-5 chapters) because I could not stand to write about a technology I did not believe in.
Beyond that, I think Dreamweaver and Expression Web are good products.
Stefan