php-关于检查文件类型函数的疑问

 /** * 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？