Overriding a webservice for the Moodle activity chooser

lib.php

/**
 * A little something for your Moodle plugin to filter the activity chooser.
 * @param object $function Details of the function to be called.
 * @param array $params Parameters passed by the ajax call.
 * @return boolean|array If the callback returns anything other than false, we assume it replaces the original function.
 */
function theme_mytheme_override_webservice_execution($function, $params) {
    if ($function->name === 'core_course_get_course_content_items') {
        $courseid = $params[0]; // This should be courseid.
        $result = call_user_func_array([$function->classname, $function->methodname], $params);
        foreach ($result['content_items'] as $index => $contentitem) {
            if (!canuse($contentitem->name, $courseid)) {
                unset($result['content_items'][$index]);
            }
        }
        return $result;
    }
    return false;
}