I'm not a PHP person, and have only a little PHP experience but:
It is possible to route it all through one page. A well-established pattern to help do this is
MVC[
^]. There are good MVC frameworks out there apparently (Google "PHP MVC"). Or you can write your own
http://anantgarg.com/2009/03/13/write-your-own-php-mvc-framework-part-1/[
^] if you really want to get to grips with this as a problem.
You can use query strings in the URL to pass information, or create generated JavaScripts to server up the correct view and do stuff.
To your second question, it is matter of horses-for-courses. If you just want several pages with different content, then I'd answer no. If you have lots of process-specific stuff (like Wizards) where the MVC pattern is a good fit, or you are just doing it to see how it would work, then I'd say yes.