Capture additional headers, and allow others through via a whitelist.

See #3.

index-redis-page-cache.php

@@ -37,8 +37,9 @@ $redis_page_cache_config = array(
 	'redis_server'       => '127.0.0.1',
 	'redis_port'         => 6379,
 	'redis_db'           => 0,
-	'cache_version'  => 1,
+	'cache_version'      => 0,
 	'cache_headers'      => true,
+	'additional_headers' => array( 'link', 'x-hacker', 'x-pingback' ),
 	'secret_string'      => 'changeme',
 );
 
@@ -275,6 +276,9 @@ if ( ! defined( 'WP_USE_THEMES' ) ) {
 	define( 'WP_USE_THEMES', true );
 }
 
+// Set a header advertising the cache engine
+header( 'X-Redis-Page-Cache: Redis Page Cache for WordPress by Erick Hitter (http://eth.pw/rpc)', true );
+
 try {
 	// Establish connection with Redis server
 	$redis = redis_page_cache_connect_redis();
@@ -311,6 +315,13 @@ try {
 		// Retrieve cached page, which is an array that includes meta data along with the page output
 		$cache = unserialize( $redis->get( $redis_page_cache_config['redis_key'] ) );
 
+		// Output cached headers from original page
+		if ( ! empty( $cache['headers'] ) ) {
+			foreach ( $cache['headers'] as $key => $value ) {
+				header( "{$key}: {$value}", true );
+			}
+		}
+
 		// Output cache headers if desired
 		if ( $redis_page_cache_config['cache_headers'] ) {
 			header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s', $cache['time'] ) . ' GMT', true );
@@ -360,12 +371,37 @@ try {
 
 				// Cache rendered page if appropriate
 				if ( ! is_404() && ! is_search() ) {
+					// Default cache payload
 					$cache = array(
 						'output'  => $output,
 						'time'    => time(),
 						'age'     => 31536000, // one year in seconds
+						'headers' => array(),
 					);
 
+					// Capture certain headers
+					// Props to @andy and Batcache (http://wordpress.org/plugins/batcache/) for this code
+					if ( ! empty( $redis_page_cache_config['additional_headers' ] ) ) {
+						if ( function_exists( 'headers_list' ) ) {
+							foreach ( headers_list() as $header ) {
+								list( $key, $value ) = array_map( 'trim', explode( ':', $header, 2 ) );
+								$cache['headers'][ $key ] = $value;
+							}
+						} elseif ( function_exists( 'apache_response_headers' ) ) {
+							$cache['headers'] = apache_response_headers();
+						}
+
+						if ( $cache['headers'] ) {
+							foreach ( $cache['headers'] as $key => $value ) {
+								if ( ! in_array( strtolower( $key ), $redis_page_cache_config['additional_headers' ] ) )
+									unset( $cache['headers'][$key] );
+							}
+						}
+
+						unset( $key );
+						unset( $value );
+					}
+
 					// Is unlimited cache life requested?
 					if ( ! isset( $redis_page_cache_config['unlimited'] ) ) {
 						$redis_page_cache_config['unlimited'] = (bool) get_option( 'redis-page-cache-debug', false );