code: private static function getDocTypeFromMode($mode)
      {
          static $docTypeList = array(
              self::MODE_HTML4_STRICT        => '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">',
              self::MODE_HTML4_TRANSITIONAL  => '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">',
              self::MODE_XHTML1_STRICT       => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">',
              self::MODE_XHTML1_TRANSITIONAL => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">',
              self::MODE_XHTML1_1            => '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">',
              self::MODE_HTML5               => '<!DOCTYPE html>',
          );
          return $docTypeList[$mode];
      }
docs: 指定されたモードに応じた DOCTYPE 宣言の文字列を返します.
  
  @param  string $mode モード (ただし detectMode() でバリデーション済み)
  @return string       DOCTYPE 宣言