initial code

404-fallback.php

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+<?php
+/**
+ * Plugin Name:     404 Fallback
+ * Plugin URI:      PLUGIN SITE HERE
+ * Description:     PLUGIN DESCRIPTION HERE
+ * Author:          YOUR NAME HERE
+ * Author URI:      YOUR SITE HERE
+ * Text Domain:     404-fallback
+ * Domain Path:     /languages
+ * Version:         0.1.0
+ *
+ * @package         404_Fallback
+ */
+
+require_once plugin_dir_path( __FILE__ ) . '/lib/menus.php';
+require_once plugin_dir_path( __FILE__ ) . '/lib/menu-site.php';
+
+function fb404_redirect_404() {
+	global $wp;
+
+	$request = wp_unslash( $_SERVER['REQUEST_URI'] );
+	$url = stripslashes( get_option( 'fb404_setting_fallback_url' ));
+	if ( is_404() && !empty($url) ) {
+		$location      =  $url . $request;
+		$status        = 302;
+		$x_redirect_by = '404 Fallback';
+		wp_redirect( $location, $status, $x_redirect_by );
+		die();
+	}
+}
+
+add_action( 'template_redirect', 'fb404_redirect_404' );
+
+/**
+ * Add a site menu called Cluster
+ */
+function fb404_add_settings_menu() {
+	wpc_menu_register_page( [
+		'id'         => 'fb404',
+		'menu_slug'  => 'fb404_page',
+		'page_title' => '404 Fallback Settings',
+		'menu_title' => '404 Fallback',
+		'action'     => 'fb404_page_update_handler',
+	] );
+
+	wpc_menu_register_settings( 'fb404', [
+		'general' => [
+			'fb404_setting_fallback_url' => 'Fallback URL',
+		],
+	] );
+	do_action( __FUNCTION__ );
+}
+
+add_action( 'init', 'fb404_add_settings_menu' );
+
+/**
+ * Templating for our menu page. The function must match the 'menu_slug' in the menu config.
+ */
+function fb404_page() {
+	// Use the default page rendering instead of templating our page here.
+	wpc_menu_page_render( 'fb404' );
+}
+
+/**
+ * Handling the form submission. The function must match the 'action' key in the menu config.
+ */
+function fb404_page_update_handler() {
+	// Use the default page update handler instead of processing our options in a custom way.
+	wpc_menu_page_update_handler( 'fb404' );
+}
+
+function fb404_setting_fallback_url_render() {
+	$option_name = 'fb404_setting_fallback_url';
+	$config      = stripslashes( get_option( $option_name ) );
+	printf( '<input name="%1$s" value="%2$s"
+	size="48"/>', $option_name, $config );
+}

lib/menu-site.php

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+<?php
+
+/**
+ * Code used by site menus.
+ */
+
+/**
+ * List of menus and their setup configuration
+ *
+ * @param string $menu_id Identifier of the menu configuration to access. Leave empty to return all the configurations
+ *
+ * @return array Menu configuration(s)
+ */
+function wpc_menu_page_configs( $menu_id = '' ) {
+	return wpc_menu_configs( 'wpc_menu_page_configs', $menu_id );
+}
+
+function wpc_menu_page_render( $menu_id ) {
+	$config = wpc_menu_page_configs( $menu_id );
+	wpc_menu_render( 'site', $config );
+}
+
+/**
+ * Registration of the setting groups to the menu_slug.
+ */
+function wpc_menu_page_settings_registration() {
+	$menus = wpc_menu_page_configs();
+	wpc_menu_registration( $menus );
+}
+
+add_action( 'admin_init', 'wpc_menu_page_settings_registration' );
+
+/**
+ * A generic way to handle the form post request.
+ *
+ * @param string $menu_id Menu page configuration id
+ */
+function wpc_menu_page_update_handler( $menu_id ) {
+	// Make sure we are posting from our options page. Note we must add the '-options' postfix
+	// when we check the referrer.
+	$config  = wpc_menu_page_configs( $menu_id );
+	$updated = wpc_menu_update_options( 'site', $config, true );
+
+	wp_redirect( add_query_arg( [
+		'page'    => $config['menu_slug'],
+		'updated' => (string) $updated,
+	], admin_url( 'admin.php' ) ) );
+	exit;
+
+}
+
+/**
+ * Register a grouped setting to a menu page id.
+ *
+ * @example Add a setting to the general settings group of the cluster menu-page:
+ *        wpc_menu_register_page_settings( 'cluster', [
+ *            'general' => [
+ *                'wpc_cluster_use_production_images' => 'Use Production Images',
+ *            ],
+ *        ] );
+ *
+ * @param string|array $menu_id Menu identifier or configuration
+ * @param array $grouped_settings Array of setting groups.
+ */
+function wpc_menu_register_settings( $menu_id, $grouped_settings ) {
+
+	add_filter( 'wpc_menu_page_configs', function ( $menus ) use ( $menu_id, $grouped_settings ) {
+		if ( empty( $menus[ $menu_id ]['setting_sections'] ) ) {
+			$menus[ $menu_id ]['setting_sections'] = [];
+		}
+		$menus[ $menu_id ]['setting_sections'] = array_merge_recursive( $menus[ $menu_id ]['setting_sections'], $grouped_settings );
+
+		return $menus;
+	}, 11 );
+
+}
+
+/**
+ * Register a menu-page.
+ *
+ * @example Add a setting to the general settings group of the cluster menu-page:
+ *        wpc_menu_register_page( [
+ *            'id'         => 'cluster',
+ *            'menu_slug'  => 'wpc_network_cluster_page',
+ *            'page_title' => 'Cluster Settings',
+ *            'menu_title' => 'Cluster',
+ * ] );
+ *
+ * The following keys are optional:
+ *   - action: points to the function that will update the options instead
+ *     of wpc_menu_page_update_handler()
+ *
+ * @param $config
+ */
+function wpc_menu_register_page( $config ) {
+	add_filter( 'wpc_menu_page_configs', function ( $menus ) use ( $config ) {
+		$menus[ $config['id'] ] = $config;
+
+		return $menus;
+	} );
+
+	$defaults = [
+		'page_title' => 'My Menu Page',
+		'menu_title' => 'My Menu',
+	];
+	$config   = wp_parse_args( $config, $defaults );
+
+	add_action( 'admin_menu', function () use ( $config ) {
+		$menu_slug = $config['menu_slug'];
+
+		if ( empty( $config['parent_slug'] ) ) {
+			add_menu_page(
+				$config['page_title'],
+				$config['menu_title'],
+				'manage_options',
+				$menu_slug,
+				$menu_slug
+			);
+		} else {
+			add_submenu_page(
+				$config['parent_slug'],
+				$config['page_title'],
+				$config['menu_title'],
+				'manage_options',
+				$menu_slug,
+				$menu_slug
+			);
+		}
+	} );
+
+	$action = $config['action'];
+
+	/**
+	 * This function here is hooked up to a special action and necessary to process the saving of the options.
+	 */
+	add_action( 'admin_post_' . $action, $action );
+}

lib/menus.php

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+<?php
+/**
+ * Code used by both networkmenus and sitemenus
+ */
+
+/**
+ * Accessor for creating menupage configurations
+ *
+ * @param string $filter_id Extension point for other code to hook menus into.
+ * @param string $menu_id Identifier of the menu to retrieve from the filter.
+ *
+ * @return array Menu configuration
+ */
+function wpc_menu_configs( $filter_id, $menu_id = '' ) {
+	/*
+	 * Amend the networkmenupage configurations
+	 * Almost always it's better to use  one of the following functions instead of calling this directly:
+	 *   - function wpc_menu_register_networkpage( $config )
+	 *   - function wpc_menu_register_network_settings( $menu_id, $grouped_settings )
+	 */
+	$menus = apply_filters( $filter_id, [] );
+
+	// All menus
+	if ( false === isset( $menus[ $menu_id ] ) ) {
+		return $menus;
+	}
+
+	return $menus[ $menu_id ];
+}
+
+/**
+ * @param $menus
+ */
+function wpc_menu_registration( array $menus ): void {
+	// for each menu's data build out sections and register the section's settings
+	foreach ( array_keys( $menus ) as $id ) {
+		$config = $menus[ $id ];
+		if ( empty( $config['setting_sections'] ) ) {
+			continue;
+		}
+		$sections = $config['setting_sections'];
+
+		if ( is_iterable( $sections ) ) {
+			foreach ( $sections as $section => $settings ) {
+				$page = $config['menu_slug'];
+				add_settings_section( $section, strtoupper( $section ), null, $page );
+
+				foreach ( (array) $settings as $name => $title ) {
+					$sanitization = is_callable( $name . '_sanitize' ) ? $name . '_sanitize' : [];
+					register_setting( $section, $name, $sanitization );
+					add_settings_field( $name, $title,
+						$name . '_render', $page,
+						$section );
+				}
+			}
+		}
+	}
+}
+
+/**
+ * Update a single option
+ *
+ * @param string $type Type: network or site
+ * @param bool $delete_missing Whether to delete the option if not submitted?
+ * @param string $option Option name
+ *
+ * @return bool False if value was not updated, true if updated.
+ */
+function wpc_menu_update_option( $type, $delete_missing, $option ): bool {
+	$updated = false;
+
+	if ( isset( $_POST[ $option ] ) ) {
+		if ( 'network' === $type ) {
+			$updated = update_site_option( $option, $_POST[ $option ] );
+		} else {
+			$updated = update_option( $option, $_POST[ $option ] );
+
+		}
+	} elseif ( $delete_missing ) {
+		if ( 'network' === $type ) {
+			$updated = delete_site_option( $option );
+		} else {
+			$updated = delete_option( $option );
+		}
+	}
+
+	return $updated;
+}
+
+/**
+ * Update the options for a given network page configuration
+ *
+ * @param string $type network|site configuration type
+ * @param array|string $config menu configuration or id of one.
+ * @param bool $delete_missing if missing options should be deleted or skipped
+ *
+ * @return bool if any of the options was updated or deleted
+ */
+function wpc_menu_update_options( $type, $config, $delete_missing ) {
+	global $new_whitelist_options;
+
+	check_admin_referer( $config['menu_slug'] . '-options' );
+
+	$did_update = false;
+
+	// Update or delete all the options
+	$sections = $config['setting_sections'];
+	if ( false === is_iterable( $sections ) ) {
+		return $did_update ? 'true' : 'false';
+	}
+	foreach ( (array) $sections as $section => $options ) {
+
+		$options = $new_whitelist_options[ $section ];
+		foreach ( (array) $options as $option ) {
+			$updated = wpc_menu_update_option( $type, $delete_missing, $option );
+			if ( $updated ) {
+				$did_update = true;
+			}
+		}
+	}
+
+	return $did_update ? 'true' : 'false';
+
+}
+
+function wpc_menu_render( $type, $config ) {
+	// Nonce is verified by wpc_menu_fields
+	if ( isset( $_GET['updated'] ) ): ?>
+		<div id="message" class="updated notice is-dismissible"><p><?php _e( 'Options saved.' ); ?></p></div>
+	<?php endif;
+
+	$full_action = [
+		'network' => 'edit.php?action=' . esc_attr( $config['action'] ),
+		'site'    => admin_url( 'admin-post.php' ),
+	];
+	?>
+	<div class="wrap custom-options">
+		<header class="custom-options__header">
+			<h1><?php echo esc_html( $config['page_title'] ); ?></h1>
+		</header>
+
+		<form action="<?php echo $full_action[ $type ] ?>" method="POST">
+			<?php
+			wpc_menu_fields( $type, $config );
+			do_settings_sections( $config['menu_slug'] );
+			submit_button();
+			?>
+		</form>
+	</div>
+	<?php
+}
+
+/**
+ * Configuration type aware settings fields
+ *
+ * @param string $type network|site
+ * @param array $config menu configuration
+ */
+function wpc_menu_fields( $type, $config ) {
+	settings_fields( $config['menu_slug'] );
+	if ( 'network' !== $type ) {
+		echo "<input type='hidden' name='action' value='" . esc_attr( $config['action'] ) . "' />";
+	}
+}