diff --git a/wp-ffpc-backend.php b/wp-ffpc-backend.php
index 408d052..41466ab 100644
--- a/wp-ffpc-backend.php
+++ b/wp-ffpc-backend.php
@@ -44,6 +44,7 @@ abstract class WP_FFPC_Backend {
 		$ruri = isset ( $_SERVER['REQUEST_URI'] ) ? $_SERVER['REQUEST_URI'] : '';
 		$rhost = isset ( $_SERVER['HTTP_HOST'] ) ? $_SERVER['HTTP_HOST'] : '';
 		$scookie = isset ( $_COOKIE['PHPSESSID'] ) ? $_COOKIE['PHPSESSID'] : '';
+		$release = defined('WP_RELEASE') ? WP_RELEASE : '';
 
 		if ( isset( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' )
 			$_SERVER['HTTPS'] = 'on';
@@ -56,6 +57,7 @@ abstract class WP_FFPC_Backend {
 			'$request_uri' => $ruri,
 			'$remote_user' => $ruser,
 			'$cookie_PHPSESSID' => $scookie,
+			'$release' => $release
 		);
 
 		/* split single line hosts entry */
diff --git a/wp-ffpc-class.php b/wp-ffpc-class.php
index 29cc836..2978956 100644
--- a/wp-ffpc-class.php
+++ b/wp-ffpc-class.php
@@ -162,6 +162,7 @@ class WP_FFPC extends WP_FFPC_ABSTRACT {
 			'$remote_user' => __('Name of user, authenticated by the Auth Basic Module', 'wp-ffpc'),
 			'$cookie_PHPSESSID' => __('PHP Session Cookie ID, if set ( empty if not )', 'wp-ffpc'),
 			'$accept_lang' => __('First HTTP Accept Lang set in the HTTP request', 'wp-ffpc'),
+			'$release' => __('A unique release number. You can set this dynamically to invalidate cache on new releases by setting the following in wp-config.php (or another included file): <pre>define("WP_RELEASE", "1.0");</pre>')
 			//'$cookie_COOKnginy IE' => __('Value of COOKIE', 'wp-ffpc'),
 			//'$http_HEADER' => __('Value of HTTP request header HEADER ( lowercase, dashes converted to underscore )', 'wp-ffpc'),
 			//'$query_string' => __('Full request URI after rewrites', 'wp-ffpc'),