I am at the new end of the Drupal spectrum. I built about five sites. I built my first few sites using little or no PHP (I used enough CSS and HTML markup). Therefore, I would say that in Drupal you can build a little understanding of PHP.
But. Drupal is a complex system that seems to be favored by developers, as opposed to end users who may favor a click-based approach (although this may change).
I found that the more I understand the code base and API, the easier it is to get what I want without having to wade through a huge number of modules. The repository of material contributed to Drupal is fantastic and covers a lot of space, but is not always actively maintained and may not do what you want.
I think a good place to start is to understand the hook system, the forms API and the menu system, as well as how to generate requests. These are more and more back things. On the front side: a template system, template.php and preprocess and theme functions.
Checkout api.drupal.org .
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