<?php
/*
Plugin Name: Redirect by Custom Field
Description: Changes the URLs pointing to pages and posts which have a "redirect" custom field, using that entry as the URL instead.
Author: mitcho (Michael 芳貴 Erlewine)
Version: 0.4
Author URI: http://mitcho.com/
Donate link: https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=66G4DATK4999L&item_name=mitcho%2ecom%2fcode%3a%20donate%20to%20Michael%20Yoshitaka%20Erlewine&no_shipping=0&no_note=1&tax=0&currency_code=USD&lc=US&charset=UTF%2d8
*/

// This section will redirect URL's when the real permalink is hit directly.
// Maybe I should have used the rewrite API or the canonical_redirect filter instead?
// Note: I didn't use rewrite because I didn't want to have to keep track of all redirect mappings.
add_action('parse_query', 'redirect_header', 10, 1);
function redirect_header($query) {
	if (is_singular()) {
		$link = redirect_by_custom_field(false, $query->queried_object);
		if ($link !== false) {
			wp_redirect(esc_url($link), 301);
			exit;
		}
	}
	return $headers;
}

// This section will replace every instance of the permalink with the redirect URL.
add_filter('page_link', 'redirect_by_custom_field', 10, 2);
add_filter('post_link', 'redirect_by_custom_field', 10, 2);
function redirect_by_custom_field($link, $postarg = null) {
	global $post;
	if (is_object($postarg))
		$id = $postarg->ID;
	else if (is_int($postarg))
		$id = $postarg;
	else if (is_object($post))
		$id = $post->ID;
	else
		return $link;
	
	$redirect = get_post_meta($id, 'redirect', true);
	if ($redirect) {
		if (is_array($redirect))
			$redirect = $redirect[0];
		$link = $redirect;
	}
	return $link;
}