What is the difference between Drupal PHP and normal PHP

If I want to modify the Drupal PHP code then this does not really look like PHP. It's different from what I have learned about PHP. Is there any online tutorial how to understand the coding style of Drupal better so that I am able to modify the Drupal Code better. I also would like to create custom modules for Drupal. Can anybody tell me how to learn the Drupal PHP coding style?

No answers yet