Airship\Engine\Security\Util::subString PHP 메소드

    public static function subString(string $str, int $start, $length = null) : string
    {
        return Binary::safeSubstr($str, $start, $length);
    }

 /**
  * @param string $mimeHeader
  * @return bool
  */
 protected function isViewable(string $mimeHeader) : bool
 {
     $pos = \strpos($mimeHeader, ';');
     if ($pos !== false) {
         $mimeHeader = Util::subString($mimeHeader, 0, $pos);
     }
     return \in_array($mimeHeader, $this->viewableMimeTypes);
 }