WordPress plugin to redirect old URLs when a site is refreshed. Uses a JSON file on activate to import into the DB. If Activation fails because of an error in the JSON file just deactivate, edit the file and then reactivate.

redirect-old-urls.php

<?php
/**
 * Plugin Name: Redirect Old URLs
 * Description: Supports redirecting from an old site's URL to a new site on same domain.
 * Version: 0.1.0
 * Author: The WPLib Team
 * Author URI: http://github.com/wplib
 * Author Email: [email protected]
 * License: GPL2
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 *
 * Looks for a redirects.json file in the WP_CONTENT_DIR directory (typically /wp-content) on plugin activation
 * that contains an array of two dimensional arrays where:
 *
 *      [0] The old URL path from the root but w/o a leading slash.
 *      [1] The new URL path to 301 redirect to.
 *
 * It then loads this into the wp_terms table where the name is prefixed with 'redirect:'.
 *
 * This will support URLs up to 200 characters in length.
 *
 * @example:
 *
 *      [
 *          [ 'old-foo/', 'new-foo/' ],
 *          [ 'old-bar/', 'new-bar/' ],
 *          ...
 *      ]
 *
 * @note Yes, this uses the wp_terms in a non-standard way.  But it is better than creating a
 *       custom table as it should not affect anything because it will be an orphan term from
 *       the perspective of WordPress as it will have no associated taxonomy and thus these 
 *       terms will be ignored. 
 *
 *       Just DO NOT RUN any plugin that DELETES ORPHAN TERMS! 
 * 
 */
class Redirect_Old_Urls {

	const REDIRECTS_FILE = '/redirects.json';

	/**
	 *
	 */
	static function on_load() {

		add_action( 'template_redirect', array( __CLASS__, '_template_redirect') );
		add_action( 'pre_current_active_plugins',  array( __CLASS__, '_pre_current_active_plugins') );
		register_activation_hook( __FILE__, array( __CLASS__, '_activate') );

	}

	/**
	 *
	 */
	static function _template_redirect() {

		if ( is_404() || is_feed() ) {

			/**
			 * Only attempt to redirect if on a 404
			 *
			 * First, get the incoming URL path, stripping it of leading and trailing slashes,
			 *
			 * @example http://example.com/foo/bar/baz/ -->  'foo/bar/baz'
			 */

			$url_path = trim( $_SERVER['REQUEST_URI'], '/\\' );

			if ( $new_path = self::lookup_url_redirect( $url_path ) ) {
				/**
				 * If we found a URL to redirect, do so.
				 */
				wp_safe_redirect( "/{$new_path}", 301 );
				die;

			}
			/**
			 * If no redirect found then continue on and let WordPress display a 404 page.
			 */
		}

	}

	/**
	 * Retrieves a new url path given an old url path from the wp_terms table.
	 *
	 * @param string|array $old_path
	 *
	 * @return string
	 */
	static function lookup_url_redirect( $old_path ) {
		/**
		 * @var wpdb $wpdb
		 */
		global $wpdb;

		$sql = $wpdb->prepare(
			"SELECT slug FROM {$wpdb->terms} WHERE name = '%s' LIMIT 1",
			"redirect:{$old_path}"
		);

		return $wpdb->get_var( $sql );

	}

	/**
	 *
	 */
	static function _activate() {

		do {

			$errors = array();

			if ( ! is_file( $redirects_filepath = self::redirects_filepath() ) ) {
				$err_msg = "Import Terminated; no redirect file found: %s.";
				$errors[] = sprintf( $err_msg, $redirects_filepath );
				break;
			}

			if ( ! ( $redirects_json = json_decode( file_get_contents( $redirects_filepath ) ) ) ) {
				$err_msg = "Import Terminated; redirects file appears to be invalid JSON: %s.";
				$errors[] = sprintf( $err_msg, $redirects_filepath );
				break;
			}

			if ( ! is_array( $redirects_json ) ) {
				$err_msg = "Import Terminated; redirects file does not return an array of redirects: %s.";
				$errors[] = sprintf( $err_msg, $redirects_filepath );
				break;
			}

			global $wpdb;
			/**
			 * Delete all the old URL redirects, if there are any...
			 */
			$sql = "DELETE FROM {$wpdb->terms} WHERE name LIKE 'redirect:%'";
			$wpdb->query( $sql );

			foreach( $redirects_json as $index => $redirect_pair ) {

				if ( ! isset( $redirect_pair[0] ) || ! isset( $redirect_pair[1] ) ) {
					$err_msg = "Redirect #%d Failed; not a valid two element array in %s.";
					$errors[] = sprintf( $err_msg, $index, $redirects_filepath );
					continue;
				}

				if ( 191 < strlen( $redirect_pair[0] ) ) {
					$err_msg  = "Redirect #%d Failed; URL path '%s' is too long in %s (max 191 chars allowed.)";
					$errors[] = sprintf( $err_msg, $index, $redirect_pair[0], $redirects_filepath );
					continue;
				}

				if ( 200 < strlen( $redirect_pair[1] ) ) {
					$err_msg  = "Redirect #%d Failed; URL path '%s' is too long in %s (max 200 chars allowed.)";
					$errors[] = sprintf( $err_msg, $index, $redirect_pair[1], $redirects_filepath );
					continue;
				}

				$redirects_json[ $index ] = $wpdb->prepare( "('%s','%s')", "redirect:{$redirect_pair[0]}", $redirect_pair[1] );

			}

			if ( count( $errors ) ) {
				break;
			}

			$sql = "INSERT INTO {$wpdb->terms} (name,slug) VALUES \n\t" . implode( ",\n\t", $redirects_json );
			$wpdb->query( $sql );


		} while ( false );

		if ( count( $errors ) ) {

			$errors = implode( '</li><li>', $errors );

			$err_msg = sprintf( '<p>Errors while attempting to activate plugin:</p><ul><li>%s</li></ul>', $errors );

			set_transient( 'rou_activate_errors', $err_msg, 30 );


		}

	}

	/**
	 *
	 */
	static function _pre_current_active_plugins() {

		do {

			$err_msg = get_transient( 'rou_activate_errors' );

			/**
			 * If  no errors, exit
			 */
			if ( ! $err_msg ) {
				break;
			}

			/**
			 * Output the error messages
			 */
			echo '<div id="rou-message" class="error below-h2">';
			echo wp_kses_post( $err_msg );
			echo '</div>';

			/**
			 * Clear the error message
			 */
			delete_transient( 'rou_activate_errors' );

			/**
			 * Unhook notices to avoid dups
			 */
			remove_action( 'pre_current_active_plugins', array( __CLASS__, __FUNCTION__ ) );

		} while ( false );

	}

	/**
	 *
	 */
	static function redirects_filepath() {
		return WP_CONTENT_DIR . self::REDIRECTS_FILE;
	}

}
Redirect_Old_Urls::on_load();

redirects.json

[
  [ "page/13",			"about-us/"],
  [ "page/42",		  "public art page (tbd)/"],
  [ "etc",		      "new-etc/"],
  [ 
    "Store this file as /wp-content/redirects.json, or wherever you changed content to.", 
    "File is only needed during activation."
  ]
]