function bbp_merge_topic_handler( $action = '' ) {

	// Bail if action is not bbp-merge-topic
	if ( 'bbp-merge-topic' !== $action ) {
		return;
	}

	// Define local variable(s)
	$source_topic_id = $destination_topic_id = 0;
	$source_topic = $destination_topic = 0;
	$subscribers = $favoriters = $replies = array();

	/** Source Topic **********************************************************/

	// Topic id
	if ( empty( $_POST['bbp_topic_id'] ) ) {
		bbp_add_error( 'bbp_merge_topic_source_id', __( '<strong>ERROR</strong>: Topic ID not found.', 'bbpress' ) );
	} else {
		$source_topic_id = (int) $_POST['bbp_topic_id'];
	}

	// Nonce check
	if ( ! bbp_verify_nonce_request( 'bbp-merge-topic_' . $source_topic_id ) ) {
		bbp_add_error( 'bbp_merge_topic_nonce', __( '<strong>ERROR</strong>: Are you sure you wanted to do that?', 'bbpress' ) );
		return;

	// Source topic not found
	} elseif ( ! $source_topic = bbp_get_topic( $source_topic_id ) ) {
		bbp_add_error( 'bbp_merge_topic_source_not_found', __( '<strong>ERROR</strong>: The topic you want to merge was not found.', 'bbpress' ) );
		return;
	}

	// Cannot edit source topic
	if ( ! current_user_can( 'edit_topic', $source_topic->ID ) ) {
		bbp_add_error( 'bbp_merge_topic_source_permission', __( '<strong>ERROR</strong>: You do not have permission to edit the source topic.', 'bbpress' ) );
		return;
	}

	/** Destination Topic *****************************************************/

	// Topic id
	if ( empty( $_POST['bbp_destination_topic'] ) ) {
		bbp_add_error( 'bbp_merge_topic_destination_id', __( '<strong>ERROR</strong>: Destination topic ID not found.', 'bbpress' ) );
	} else {
		$destination_topic_id = (int) $_POST['bbp_destination_topic'];
	}

	// Destination topic not found
	if ( ! $destination_topic = bbp_get_topic( $destination_topic_id ) ) {
		bbp_add_error( 'bbp_merge_topic_destination_not_found', __( '<strong>ERROR</strong>: The topic you want to merge to was not found.', 'bbpress' ) );
	}

	// Cannot edit destination topic
	if ( ! current_user_can( 'edit_topic', $destination_topic->ID ) ) {
		bbp_add_error( 'bbp_merge_topic_destination_permission', __( '<strong>ERROR</strong>: You do not have permission to edit the destination topic.', 'bbpress' ) );
	}

	// Bail if errors
	if ( bbp_has_errors() ) {
		return;
	}

	/** No Errors *************************************************************/

	// Update counts, etc...
	do_action( 'bbp_merge_topic', $destination_topic->ID, $source_topic->ID );

	/** Date Check ************************************************************/

	// Check if the destination topic is older than the source topic
	if ( strtotime( $source_topic->post_date ) < strtotime( $destination_topic->post_date ) ) {

		// Set destination topic post_date to 1 second before source topic
		$destination_post_date = date( 'Y-m-d H:i:s', strtotime( $source_topic->post_date ) - 1 );

		// Update destination topic
		wp_update_post( array(
			'ID'            => $destination_topic_id,
			'post_date'     => $destination_post_date,
			'post_date_gmt' => get_gmt_from_date( $destination_post_date )
		) );
	}

	/** Engagements ***********************************************************/

	// Get engagements from source topic
	$engagements = bbp_get_topic_engagements( $source_topic->ID );

	// Maybe migrate engagements
	if ( ! empty( $engagements ) ) {
		foreach ( $engagements as $engager ) {
			bbp_add_user_engagement( $engager, $destination_topic->ID );
		}
	}

	/** Subscriptions *********************************************************/

	// Get subscribers from source topic
	$subscribers = bbp_get_subscribers( $source_topic->ID );

	// Maybe migrate subscriptions
	if ( ! empty( $subscribers ) && ! empty( $_POST['bbp_topic_subscribers'] ) && ( '1' === $_POST['bbp_topic_subscribers'] ) ) {
		foreach ( $subscribers as $subscriber ) {
			bbp_add_user_subscription( $subscriber, $destination_topic->ID );
		}
	}

	/** Favorites *************************************************************/

	// Get favoriters from source topic
	$favoriters = bbp_get_topic_favoriters( $source_topic->ID );

	// Maybe migrate favorites
	if ( ! empty( $favoriters ) && ! empty( $_POST['bbp_topic_favoriters'] ) && ( '1' === $_POST['bbp_topic_favoriters'] ) ) {
		foreach ( $favoriters as $favoriter ) {
			bbp_add_user_favorite( $favoriter, $destination_topic->ID );
		}
	}

	/** Tags ******************************************************************/

	// Get the source topic tags
	$source_topic_tags = wp_get_post_terms( $source_topic->ID, bbp_get_topic_tag_tax_id(), array( 'fields' => 'names' ) );

	// Tags to possibly merge
	if ( ! empty( $source_topic_tags ) && ! is_wp_error( $source_topic_tags ) ) {

		// Shift the tags if told to
		if ( ! empty( $_POST['bbp_topic_tags'] ) && ( "1" === $_POST['bbp_topic_tags'] ) ) {
			wp_set_post_terms( $destination_topic->ID, $source_topic_tags, bbp_get_topic_tag_tax_id(), true );
		}

		// Delete the tags from the source topic
		wp_delete_object_term_relationships( $source_topic->ID, bbp_get_topic_tag_tax_id() );
	}

	/** Source Topic **********************************************************/

	// Status
	bbp_open_topic( $source_topic->ID );

	// Sticky
	bbp_unstick_topic( $source_topic->ID );

	// Delete source topic's last & count meta data
	delete_post_meta( $source_topic->ID, '_bbp_last_reply_id'      );
	delete_post_meta( $source_topic->ID, '_bbp_last_active_id'     );
	delete_post_meta( $source_topic->ID, '_bbp_last_active_time'   );
	delete_post_meta( $source_topic->ID, '_bbp_voice_count'        );
	delete_post_meta( $source_topic->ID, '_bbp_reply_count'        );
	delete_post_meta( $source_topic->ID, '_bbp_reply_count_hidden' );

	// Delete source topics user relationships
	delete_post_meta( $source_topic->ID, '_bbp_favorite'     );
	delete_post_meta( $source_topic->ID, '_bbp_subscription' );
	delete_post_meta( $source_topic->ID, '_bbp_engagement'   );

	// Get the replies of the source topic
	$replies = (array) get_posts( array(
		'post_parent'    => $source_topic->ID,
		'post_type'      => bbp_get_reply_post_type(),
		'posts_per_page' => -1,
		'order'          => 'ASC'
	) );

	// Prepend the source topic to its replies array for processing
	array_unshift( $replies, $source_topic );

	if ( ! empty( $replies ) ) {

		/** Merge Replies *****************************************************/

		// Change the post_parent of each reply to the destination topic id
		foreach ( $replies as $reply ) {

			// Update the reply
			wp_update_post( array(
				'ID'          => $reply->ID,
				'post_title'  => '',
				'post_name'   => false,
				'post_type'   => bbp_get_reply_post_type(),
				'post_parent' => $destination_topic->ID,
				'guid'        => ''
			) );

			// Adjust reply meta values
			bbp_update_reply_topic_id( $reply->ID, $destination_topic->ID                           );
			bbp_update_reply_forum_id( $reply->ID, bbp_get_topic_forum_id( $destination_topic->ID ) );

			// Update the reply position
			bbp_update_reply_position( $reply->ID );

			// Do additional actions per merged reply
			do_action( 'bbp_merged_topic_reply', $reply->ID, $destination_topic->ID );
		}
	}

	/** Successful Merge ******************************************************/

	// Update topic's last meta data
	bbp_update_topic_last_reply_id   ( $destination_topic->ID );
	bbp_update_topic_last_active_id  ( $destination_topic->ID );
	bbp_update_topic_last_active_time( $destination_topic->ID );

	// Send the post parent of the source topic as it has been shifted
	// (possibly to a new forum) so we need to update the counts of the
	// old forum as well as the new one
	do_action( 'bbp_merged_topic', $destination_topic->ID, $source_topic->ID, $source_topic->post_parent );

	// Redirect back to new topic
	bbp_redirect( bbp_get_topic_permalink( $destination_topic->ID ) );
}