Modifies Redirection plugin behavior to only redirect on 404s

class-four-oh-four-only.php

<?php
/**
 * Plugin Name: Redirects - 404 Only
 * Description: Modifies Redirection plugin to only redirect on 404 errors
 * Version: 1.0.0
 * Author: Utkarsh
 * 
 * This mu-plugin ensures that the Redirection plugin only performs
 * redirects when a 404 error occurs, not on existing pages.
 */

namespace Utkarsh\Redirects;

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Class FourOhFourOnly
 * 
 * Modifies Redirection plugin behavior to only redirect on 404s
 */
class FourOhFourOnly {

	/**
	 * Initialize the plugin modifications
	 */
	public static function init() {
		// Disable normal redirect behavior.
		add_action( 'init', [ self::class, 'disable_normal_redirects' ], 1 );

		// Re-enable redirects only for 404s.
		add_action( 'template_redirect', [ self::class, 'handle_404_redirects' ], 5 );
	}

	/**
	 * Disable the Redirection plugin's normal redirect behavior
	 *
	 * @return void
	 */
	public static function disable_normal_redirects() {
		global $wp_filter;

		if ( ! isset( $wp_filter['init'] ) ) {
			return;
		}

		foreach ( $wp_filter['init']->callbacks as $priority => $hooks ) {
			foreach ( $hooks as $hook_key => $hook ) {
				if ( self::is_redirection_init_hook( $hook ) ) {
					remove_action( 'init', $hook['function'], $priority );
				}
			}
		}
	}

	/**
	 * Check if a hook is the Redirection plugin's init method.
	 *
	 * @param array $hook The hook to check.
	 *
	 * @return bool
	 */
	private static function is_redirection_init_hook( $hook ) {
		return is_array( $hook['function'] ) &&
				is_object( $hook['function'][0] ) &&
				get_class( $hook['function'][0] ) === 'WordPress_Module' &&
				$hook['function'][1] === 'init';
	}

	/**
	 * Handle redirects only when a 404 occurs
	 *
	 * @return void
	 */
	public static function handle_404_redirects() {
		// Only proceed if this is a 404.
		if ( ! is_404() ) {
			return;
		}

		// Check if Redirection plugin classes exist.
		if ( ! class_exists( 'Red_Url_Request' ) || ! class_exists( 'Redirection_Request' ) ) {
			return;
		}

		self::process_404_redirect();
	}

	/**
	 * Process the redirect for a 404 page
	 *
	 * @return void
	 */
	private static function process_404_redirect() {
		// Get the current request URL.
		$request = new \Red_Url_Request( \Redirection_Request::get_request_url() );

		// Make sure it's a valid URL and not protected.
		if ( ! $request->is_valid() || $request->is_protected_url() ) {
			return;
		}

		// Get all redirects that match the URL.
		$redirects = \Red_Item::get_for_url( $request->get_decoded_url() );

		// Run through the redirects until one fires.
		foreach ( (array) $redirects as $item ) {
			$action = $item->get_match(
				$request->get_decoded_url(),
				$request->get_original_url()
			);

			if ( $action ) {
				// Log the redirect if needed.
				do_action(
					'redirection_matched',
					$request->get_decoded_url(),
					$item,
					$redirects
				);

				// Perform the redirect.
				$action->run();
				exit; // Stop execution after redirect.
			}
		}
	}
}

// Initialize the plugin modifications.
FourOhFourOnly::init();