You should purchase the latest version of Dreamweaver, which will help you in the creation of Responsive websites. Responsive websites started coming into fashion about three years ago and will work as well on cell phones and tablets as they do on desktop browsers. PhoneGap is something you should jettison from your consideration due to that.
You write a website once, it works for all platforms. There is no need to develop a separate mobile site.
As to reusable code, Dreamweaver supports code snippits. Need to describe something or insert rounded corners in CSS3? No problem, insert your code snippit, reuse the code you developed some time ago and you're all set.
Cranking out lots of pages: Dreamweaver makes it easy if you wish to use the Dreamweaver templates. These are HTML templates that will allow you to write your navigation, header and footer once and have that appear "automagically" on all pages you create based on that template. Changes to your navigation, footer, header and any other element you create that is not editable in the template are updated whenever you update your template.
Additionally, since you know php, you can use server-side includes to make areas of a web page universal. Downside of doing a site with server-side includes is that you must have a testing server to actually see what you're doing and that you need to flip back and forth between Dreamweaver's Live View and Live Code to see your work (alternatively, you can have a browser open and refresh that every time you save.
You should learn the general ideas involved in HTML5, even though that specification is not adopted yet. HTML5 has more semantic elements that will help you create repeatable code that allows you to handle your style sheet, as well as blast through your HTML creation much faster. I find that HTML5's biggest advantage is the semantic elements that go beyond basic <body> <head> and <html>. With HTML5 you have <section>, <article>, <header>, <footer>, <aside> and so on. When you close these elements, you don't have a generic </div> closure—you can see what you're closing.
Don't forget that, for older browsers, you'll need to insert the HTML5 Shiv in the <head> region of your website if you're using these new selectors. Please see: https://code.google.com/p/html5shiv/
When I am developing, I generally have Dreamweaver and Photoshop open at the same time, as well as two or three dissimilar browsers. I also check for compliance with the W3C Validation Tool: http://validator.w3.org/
That should get you started.
Look at Photoshop for creating your design comps and web images.
Look at Edge Reflow for responsive layout.
Bring that project into DW for coding.
For the dynamic side of things, look at DMXZone's extensions for DW.
Or, if you're not already using it, CodeIgniter