PHP

Drupal is written in the PHP programming language. PHP is a widely supported, multiplatform, and web-centric scripting language. Since Drupal is written in PHP, this book will largely feature code written in PHP, albeit with Drupal standard practices being kept in mind.

It is very important to note that the minimum version of PHP required for Drupal 8 to run (and install via Composer) is 5.5.9. Moreover, since the current version of PHP (at the time of writing this book) is PHP 7, I personally recommend that you run Drupal 8. It's best to start off right.

Regarding the style of PHP, a very important change compared to Drupal 7 is the heavy use of object-oriented code and design patterns. Granted, many procedural style approaches remain throughout the Drupal 8 code base, but the use of a good number of popular external libraries (such as Symfony components) has pushed the overall Drupal code to be more modern. For this reason, it is also quite important that you have at least some basic understanding of object-oriented programming (OOP), especially PHP related, if you want to do Drupal 8 development.