Plugin activation hooks

Attention all pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the Plugin Directory or can be cost-based plugin from a third-party authors:

If you were using register_activation_hook() to also handle updates from older versions of your plugins, you will not be able to do so any more in WP 3.1: [16012]

The activation hook is now fired only when the user activates the plugin and not when an automatic plugin update occurs. This is consistent with how the deactivation hook works.

There is a proposal for a register_update_hook() instead: #14912

Also, the callbacks for de/activation can now accept a networknetwork (versus site, blog) activation flag: