public function mt_getPostCategories( $args ) {
		$this->escape( $args );

		$post_ID  = (int) $args[0];
		$username = $args[1];
		$password = $args[2];

		$user = $this->login( $username, $password );
		if ( ! $user ) {
			return $this->error;
		}

		if ( ! get_post( $post_ID ) ) {
			return new IXR_Error( 404, __( 'Invalid post ID.' ) );
		}

		if ( ! current_user_can( 'edit_post', $post_ID ) ) {
			return new IXR_Error( 401, __( 'Sorry, you are not allowed to edit this post.' ) );
		}

		/** This action is documented in wp-includes/class-wp-xmlrpc-server.php */
		do_action( 'xmlrpc_call', 'mt.getPostCategories' );

		$categories = array();
		$catids     = wp_get_post_categories( intval( $post_ID ) );
		// first listed category will be the primary category
		$isPrimary = true;
		foreach ( $catids as $catid ) {
			$categories[] = array(
				'categoryName' => get_cat_name( $catid ),
				'categoryId'   => (string) $catid,
				'isPrimary'    => $isPrimary,
			);
			$isPrimary    = false;
		}

		return $categories;
	}