404-301-redirect-old-url.php

<?php
	
/*
 * When migrating an old website to WordPress, you should store pages/posts/... URLs in a custom field
 * Example: /studio.php
 * You can create a metabox to manually insert the URL, on the post edit scree, or have it imported directly if you are exporting/importing from the old CMS to WordPress
 * If some visitor lands on the old URL, you then hook into the template_redirect, check if it's a 404, and then try to redirect him to the correct page/post/...
 * This is of extreme importance SEO wise
*/

add_action( 'template_redirect', 'old_urls_redirect' );

function old_urls_redirect() {
	if ( is_404() ) {
		global $wpdb;
		//Replace meta_key with your custom field name
		$post_id = $wpdb->get_var( $wpdb->prepare( "SELECT post_id FROM ".$wpdb->postmeta." WHERE meta_key='ma-404-301-old-url' AND meta_value = %s", trim( $_SERVER['REQUEST_URI'] ) ) );
		if ( $post_id ) {
			//Redirect if a post is found
			wp_redirect( get_permalink($post_id), 301 );
		} else {
			//No post found - Maybe it's an URL that do not map directly to post (Thanks Mike Schinkel)
			//I'm using an array, but we could use any other method, for example, a CPT with two custom fields "old_url" / "new_url"
			$url_mapping = array(
				//'old_url'	=>	'new_url',
			)
			if ( isset( $url_mapping[ trim( $_SERVER['REQUEST_URI'] ) ] ) && trim( $url_mapping[ trim( $_SERVER['REQUEST_URI'] ) ] ) != '' ) {
				wp_redirect( trim( $url_mapping[ trim( $_SERVER['REQUEST_URI'] ) ] ), 301 );
			}
		}
	}
}