Easily find the course id for any page

Here is a nice code snippet to find the course id of a page (assuming it's actually related to a course).

global $PAGE;

$context = $PAGE->context;
$coursecontext = $context->get_course_context();

// current course id
$courseid = $coursecontext->instanceid;

Tada! Came across this on stackoverflow today and thought I would keep a record of it for myself :)