Object-oriented PHP utilizes classes and objects to organize code into reusable chunks. This approach helps us organize complex applications, such as Drupal, into modular code called classes that can be reused across the entire system.

Caching in Drupal is controlled by Drupal's Cache API which is used to store any type of data on a permanent or temporary basis. Some types of data tend to take a long time to compute, but utilizing the Cache API in your module can help your site load data more quickly.

Performance and scalability determine how fast your application can serve a page and to how many users at a time.

Symfony is a set of reusable PHP components, and a framework for building PHP applications. Drupal makes use of various Symfony components.