php IP Get City API (Innocent IP Database)

/**
* Get city name based on IP address Innocence IP database
* edit: bbs.it-home.org
*/
 function convertip($ip) {
 //IP data file path
 $dat_path = 'QQWry.Dat';

 //Check IP address
 if(!filter_var($ip, FILTER_VALIDATE_IP)) {
 return 'IP Address Error';
 }
 //Open IP data file
 if(!$fd = @fopen($dat_path, 'rb')){
 return 'IP date file not exists or access denied';
 }

 //Decompose the IP for operation and get an integer number
 $ipNum = ip2long($ip);

 //Get the start and end position of the IP data index
 $DataBegin = fread($fd, 4);
 $DataEnd = fread($fd, 4);
 $ipbegin = implode('', unpack('L', $DataBegin));
 if($ipbegin  $ipend = implode('', unpack('L', $DataEnd));
 if($ipend $ ipNum || $ip2num $Middle= intval(($EndNum + $BeginNum) / 2);

 //Offset the pointer to the index position and read 4 bytes
 fseek($fd, $ ipbegin + 7 * $Middle);
 $ipData1 = fread($fd, 4);
 if(strlen($ipData1)  fclose($fd);
 return 'System Error';
 }
 //The extracted data is converted into a long integer. If the data is a negative number, add 2 to the 32nd power
 $ip1num = implode('', unpack('L', $ipData1));
 if($ip1num  $ipNum) {
 $EndNum = $Middle;
 continue;
 }

 //Get the next index after fetching the previous index
 $DataSeek = fread($fd, 3);
 if(strlen($DataSeek)  fclose($fd);
 return 'System Error';
 }
 $DataSeek = implode('', unpack('L', $DataSeek.chr(0)));
 fseek($fd, $DataSeek);
 $ipData2 = fread( $fd, 4);
 if(strlen($ipData2)  fclose($fd);
 return 'System Error';
 }
 $ip2num = implode('', unpack('L', $ipData2));
 if($ip2num
 //Unknown prompt not found
 if($ip2num  if($Middle = = $BeginNum) {
 fclose($fd);
 return 'Unknown';
 }
 $BeginNum = $Middle;
 }
 }

 //The following code is confusing to read. I don’t understand it. I’m interested. Slow reading
 $ipFlag = fread($fd, 1);
 if($ipFlag == chr(1)) {
 $ipSeek = fread($fd, 3);
 if(strlen($ipSeek)  fclose($fd);
 return 'System Error';
 }
 $ipSeek = implode('', unpack('L', $ipSeek.chr(0)));
 fseek($fd, $ ipSeek);
 $ipFlag = fread($fd, 1);
 }

 if($ipFlag == chr(2)) {
 $AddrSeek = fread($fd, 3);
 if(strlen($AddrSeek )  fclose($fd);
 return 'System Error';
 }
 $ipFlag = fread($fd, 1);
 if($ipFlag == chr(2)) {
 $AddrSeek2 = fread($fd, 3);
 if(strlen($AddrSeek2)  fclose($fd);
 return 'System Error';
 }
 $AddrSeek2 = implode('', unpack(' L', $AddrSeek2.chr(0)));
 fseek($fd, $AddrSeek2);
 } else {
 fseek($fd, -1, SEEK_CUR);
 }

 while(($char = fread ($fd, 1)) != chr(0))
 $ipAddr2 .= $char;

 $AddrSeek = implode('', unpack('L', $AddrSeek.chr(0)));
 fseek ($fd, $AddrSeek);

 while(($char = fread($fd, 1)) != chr(0))
 $ipAddr1 .= $char;
 } else {
 fseek($fd, - 1, SEEK_CUR);
 while(($char = fread($fd, 1)) != chr(0))
 $ipAddr1 .= $char;

 $ipFlag = fread($fd, 1);
 if ($ipFlag == chr(2)) {
 $AddrSeek2 = fread($fd, 3);
 if(strlen($AddrSeek2)  fclose($fd);
 return 'System Error';
 }
 $AddrSeek2 = implode('', unpack('L', $AddrSeek2.chr(0)));
 fseek($fd, $AddrSeek2);
 } else {
 fseek($fd, -1, SEEK_CUR);
 }
 while(($char = fread($fd, 1)) != chr(0)){
 $ipAddr2 .= $char;
 }
 }
    fclose($fd);

    //最后做相应的替换操作后返回结果
    if(preg_match('/http/i', $ipAddr2)) {
        $ipAddr2 = '';
    }
    $ipaddr = "$ipAddr1 $ipAddr2";
    $ipaddr = preg_replace('/CZ88.Net/is', '', $ipaddr);
    $ipaddr = preg_replace('/^s*/is', '', $ipaddr);
    $ipaddr = preg_replace('/s*$/is', '', $ipaddr);
    if(preg_match('/http/i', $ipaddr) || $ipaddr == '') {
        $ipaddr = 'Unknown';
    }

    return $ipaddr;
}
header("Content-type: text/html; charset=utf-8");
$ip =  $_SERVER['HTTP_X_FORWARDED_FOR'];
if(isset($_GET['q'])){
    $ip = $_GET['q'];
    if(filter_var("http://".$ip,FILTER_VALIDATE_URL,FILTER_FLAG_HOST_REQUIRED)){
        $ip = gethostbyname($ip);
    }
}
echo "$ip,".mb_convert_encoding(convertip($ip),"utf-8","gb2312");

?>