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/*
 * Modify HTTP header
 */
function lastmode_headers() {

	if(is_singular()) {
		global $post;
		$LastModified = get_the_modified_time("D, d M Y H:i:s", $post->id);

		$IfModifiedSince = false;
		if (isset($_ENV['HTTP_IF_MODIFIED_SINCE']))
			$IfModifiedSince = strtotime(substr($_ENV['HTTP_IF_MODIFIED_SINCE'], 5));  
		if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
			$IfModifiedSince = strtotime(substr($_SERVER['HTTP_IF_MODIFIED_SINCE'], 5));
		if ($IfModifiedSince && $IfModifiedSince >= $LastModified_unix) {
			header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
			exit;
		}
		header('Last-Modified: '. $LastModified);  
	}
}
add_action('template_redirect', 'lastmode_headers');