[support] Some questions re custom PHP pages in Drupal

[Donn]

Hi,
 The one thing keeping me from diving into Drupal (and I have been searching 
around for info) is:

Can I use the tools it provides (like login and templates etc.) to create 
a 'section' on a website where my own PHP code runs a mini-application like a 
shopping-basket (browse, filter, add, view, checkout) or something like a 
track-a-parcel system and so on? 
 In other words, a PHP controlled set of pages which uses a db and forms 
across many pages - the sort of thing that gets custom written for a 
particular client.

If I can 'drop in' such custom extensions to Drupal and have it do the theming 
and login control etc. then it makes sense to go with Drupal. 

If I have to place a link "click here to track your orders" (for example) and 
that link goes to another folder (call it an "external" site) where my PHP 
system runs and I must fake the look of the main website (which was under 
Drupal control) for this new area, as well as implement a new login of some 
kind -- then I may as well do everything in PHP.

Thanks,
\d