关于检查文件类型函数的疑问

Question

 /**
 * 检查文件类型
 *
 * @access      public
 * @param       string      filename            文件名
 * @param       string      realname            真实文件名
 * @param       string      limit_ext_types     允许的文件类型
 * @return      string
 */
function check_file_type($filename, $realname = '', $limit_ext_types = '')
{
    if ($realname)
    {
        $extname = strtolower(substr($realname, strrpos($realname, '.') + 1));//获取文件后缀名
    }
    else
    {
        $extname = strtolower(substr($filename, strrpos($filename, '.') + 1));
    }

    if ($limit_ext_types && stristr($limit_ext_types, '|' . $extname . '|') === false)//??
    {
        return '';
    }

    $str = $format = '';

    $file = @fopen($filename, 'rb');
    if ($file)
    {
        $str = @fread($file, 0x400); // 读取前 1024 个字节
        @fclose($file);
    }
    else
    {
        if (stristr($filename, ROOT_PATH) === false)
        {
            if ($extname == 'jpg' || $extname == 'jpeg' || $extname == 'gif' || $extname == 'png' || $extname == 'doc' ||
                $extname == 'xls' || $extname == 'txt'  || $extname == 'zip' || $extname == 'rar' || $extname == 'ppt' ||
                $extname == 'pdf' || $extname == 'rm'   || $extname == 'mid' || $extname == 'wav' || $extname == 'bmp' ||
                $extname == 'swf' || $extname == 'chm'  || $extname == 'sql' || $extname == 'cert'|| $extname == 'pptx' || 
                $extname == 'xlsx' || $extname == 'docx')
            {
                $format = $extname;
            }
        }
        else
        {
            return '';
        }
    }

    if ($format == '' && strlen($str) >= 2 )
    {
        if (substr($str, 0, 4) == 'MThd' && $extname != 'txt')
        {
            $format = 'mid';
        }
        elseif (substr($str, 0, 4) == 'RIFF' && $extname == 'wav')
        {
            $format = 'wav';
        }
        elseif (substr($str ,0, 3) == "\xFF\xD8\xFF")
        {
            $format = 'jpg';
        }
        elseif (substr($str ,0, 4) == 'GIF8' && $extname != 'txt')
        {
            $format = 'gif';
        }
        elseif (substr($str ,0, 8) == "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A")
        {
            $format = 'png';
        }
        elseif (substr($str ,0, 2) == 'BM' && $extname != 'txt')
        {
            $format = 'bmp';
        }
        elseif ((substr($str ,0, 3) == 'CWS' || substr($str ,0, 3) == 'FWS') && $extname != 'txt')
        {
            $format = 'swf';
        }
        elseif (substr($str ,0, 4) == "\xD0\xCF\x11\xE0")
        {   // D0CF11E == DOCFILE == Microsoft Office Document
            if (substr($str,0x200,4) == "\xEC\xA5\xC1\x00" || $extname == 'doc')
            {
                $format = 'doc';
            }
            elseif (substr($str,0x200,2) == "\x09\x08" || $extname == 'xls')
            {
                $format = 'xls';
            } elseif (substr($str,0x200,4) == "\xFD\xFF\xFF\xFF" || $extname == 'ppt')
            {
                $format = 'ppt';
            }
        } elseif (substr($str ,0, 4) == "PK\x03\x04")
        {
            if (substr($str,0x200,4) == "\xEC\xA5\xC1\x00" || $extname == 'docx')
            {
                $format = 'docx';
            }
            elseif (substr($str,0x200,2) == "\x09\x08" || $extname == 'xlsx')
            {
                $format = 'xlsx';
            } elseif (substr($str,0x200,4) == "\xFD\xFF\xFF\xFF" || $extname == 'pptx')
            {
                $format = 'pptx';
            }else
            {
                $format = 'zip';
            }
        } elseif (substr($str ,0, 4) == 'Rar!' && $extname != 'txt')
        {
            $format = 'rar';
        } elseif (substr($str ,0, 4) == "\x25PDF")
        {
            $format = 'pdf';
        } elseif (substr($str ,0, 3) == "\x30\x82\x0A")
        {
            $format = 'cert';
        } elseif (substr($str ,0, 4) == 'ITSF' && $extname != 'txt')
        {
            $format = 'chm';
        } elseif (substr($str ,0, 4) == "\x2ERMF")
        {
            $format = 'rm';
        } elseif ($extname == 'sql')
        {
            $format = 'sql';
        } elseif ($extname == 'txt')
        {
            $format = 'txt';
        }
    }

    if ($limit_ext_types && stristr($limit_ext_types, '|' . $format . '|') === false)
    {
        $format = '';
    }

    return $format;
}

整个函数用于检测文件类型，其中

 elseif (substr($str ,0, 3) == "\xFF\xD8\xFF")
        {
            $format = 'jpg';
        }


                 elseif (substr($str ,0, 8) == "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A")
        {
            $format = 'png';
        }

                 elseif (substr($str ,0, 4) == "\xD0\xCF\x11\xE0")


                   if (substr($str,0x200,4) == "\xEC\xA5\xC1\x00" || $extname == 'doc')
            {
                $format = 'doc';
            }
            elseif (substr($str,0x200,2) == "\x09\x08" || $extname == 'xls')
            {
                $format = 'xls';
            } elseif (substr($str,0x200,4) == "\xFD\xFF\xFF\xFF" || $extname == 'ppt')
            {
                $format = 'ppt';
            }

这些语句的原理是什么？而且为什么都用十六进制？

Answer 1

写在文件内容开始处的标识是数字，是 16 或多少机制没有关系，关键是数值正确就行！