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$LastModified_unix = strtotime(date("D, d M Y H:i:s", filectime($_SERVER['SCRIPT_FILENAME']))); 
$LastModified = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
 $IfModifiedSince = false; 
if (isset($_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);