I’m using Drupal 6 and its core comment module.
I want to run a custom funcion every time a user posts a comment, but I don’t want to mess with the comment.module for obvious reasons.
Is there a practical way I can do this in a custom module? If not, what are the alternatives as I don’t want to write my own commenting system.
The Drupal core api provides a hook for interacting with comments.
See http://api.drupal.org/api/function/hook_comment
Implement hook_comment in your custom module’s .module file.
For example: