Drupal Hooks: A Brief Introduction

Hooks in Drupal are a way of triggering code to run based on naming convention.

This simple and powerful system is a key way in which you or other developers can extend Drupal.

http://api.drupal.org/api/group/hooks/7

Drupal's module system is based on the concept of "hooks". A hook is a PHP function that is named foo_bar(), where "foo" is the name of the module (whose filename is thus foo.module) and "bar" is the name of the hook. Each hook has a defined set of parameters and a specified result type.

To extend Drupal, a module need simply implement a hook. When Drupal wishes to allow intervention from modules, it determines which modules implement a hook and calls that hook in all enabled modules that implement it.

Hooks in D7 have undergone a process of de-opping, meaning that where one hook was called multiple times in different contexts (by way of different 'op' values being passed in), now a separate hook runs for each case. (This may be less convenient in certain cases, but overall it is a major win for the coding experience.