code: public static function getHeader($header, $defaultValue = '')
      {
          $header = strtolower(str_replace(['_', ' '], '-', $header));
          $headers = self::getHeaders();
          if (is_array($headers)) {
              foreach ($headers as $name => $value) {
                  if (strtolower($name) == $header) {
                      return $value;
                  }
              }
          }
          return $defaultValue;
      }
docs: Get header.
  
  @param string $header
  @param string $defaultValue
  @return mixed|string