protected function getIfModifiedSinceHeader()
{
$modifiedSince = null;
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
$modifiedSince = $_SERVER['HTTP_IF_MODIFIED_SINCE'];
if (false !== ($semicolonPos = strpos($modifiedSince, ';'))) {
$modifiedSince = substr($modifiedSince, 0, $semicolonPos);
}
}
return $modifiedSince;
}