[wp-hackers] Checking if a plugin exists

[Andrew Nacin]

if (is_plugin_active('plugin-directory/plugin-file.php')) {
   //plugin is activated
}

That is generally a fine solution, though there are two others:

1. Checking for function or class existence is also a solid solution, one I
would probably use over is_plugin_active().

2. If you're building plugins that depend on others, then it is best to
attach the initialization of the sub-plugin to a hook in the parent plugin.
Then the sub-plugin will noop if the parent plugin is not activated.
BuddyPress does this. Of course, that means the parent plugin needs to have
hooks in place for that.