bbp_subscriptions_handler( string $action = '' )

function bbp_subscriptions_handler( $action = '' ) {

	if ( !bbp_is_subscriptions_active() ) {
		return false;
	}

	// Bail if no topic ID is passed
	if ( empty( $_GET['topic_id'] ) ) {
		return;
	}

	// Setup possible get actions
	$possible_actions = array(
		'bbp_subscribe',
		'bbp_unsubscribe',
	);

	// Bail if actions aren't meant for this function
	if ( !in_array( $action, $possible_actions ) ) {
		return;
	}

	// Get required data
	$user_id  = bbp_get_user_id( 0, true, true );
	$topic_id = intval( $_GET['topic_id'] );

	// Check for empty topic
	if ( empty( $topic_id ) ) {
		bbp_add_error( 'bbp_subscription_topic_id', __( '<strong>ERROR</strong>: No discussion was found! Which discussion are you subscribing/unsubscribing to?', 'buddyboss' ) );

	// Check nonce
	} elseif ( ! bbp_verify_nonce_request( 'toggle-subscription_' . $topic_id ) ) {
		bbp_add_error( 'bbp_subscription_topic_id', __( '<strong>ERROR</strong>: Are you sure you wanted to do that?', 'buddyboss' ) );

	// Check current user's ability to edit the user
	} elseif ( !current_user_can( 'edit_user', $user_id ) ) {
		bbp_add_error( 'bbp_subscription_permissions', __( '<strong>ERROR</strong>: You don\'t have the permission to edit favorites of that user!', 'buddyboss' ) );
	}

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

	/** No errors *************************************************************/

	$is_subscription = bbp_is_user_subscribed( $user_id, $topic_id );
	$success         = false;

	if ( true === $is_subscription && 'bbp_unsubscribe' === $action ) {
		$success = bbp_remove_user_subscription( $user_id, $topic_id );
	} elseif ( false === $is_subscription && 'bbp_subscribe' === $action ) {
		$success = bbp_add_user_subscription( $user_id, $topic_id );
	}

	// Do additional subscriptions actions
	do_action( 'bbp_subscriptions_handler', $success, $user_id, $topic_id, $action );

	// Success!
	if ( true === $success ) {

		// Redirect back from whence we came
		if ( bbp_is_subscriptions() ) {
			$redirect = bbp_get_subscriptions_permalink( $user_id );
		} elseif ( bbp_is_single_user() ) {
			$redirect = bbp_get_user_profile_url();
		} elseif ( is_singular( bbp_get_topic_post_type() ) ) {
			$redirect = bbp_get_topic_permalink( $topic_id );
		} elseif ( is_single() || is_page() ) {
			$redirect = get_permalink();
		} else {
			$redirect = get_permalink( $topic_id );
		}

		wp_safe_redirect( $redirect );

		// For good measure
		exit();

	// Fail! Handle errors
	} elseif ( true === $is_subscription && 'bbp_unsubscribe' === $action ) {
		bbp_add_error( 'bbp_unsubscribe', __( '<strong>ERROR</strong>: There was a problem unsubscribing from that discussion!', 'buddyboss' ) );
	} elseif ( false === $is_subscription && 'bbp_subscribe' === $action ) {
		bbp_add_error( 'bbp_subscribe',    __( '<strong>ERROR</strong>: There was a problem subscribing to that discussion!', 'buddyboss' ) );
	}
}