PHP根据IP地址获取所在城市具体实现

/**
     * 根据ip地址获取对应所在城市
     * @param type $userip 用户IP地址
     * @return string
     */
    public function getCity( $userip, $dat_path = '' ) {
        //IP数据库路径，这里用的是QQ IP数据库 20110405 纯真版
        empty( $dat_path ) && $dat_path = FCPATH . 'plugin/ipLocation/qqwry/QQWry.Dat';
        //判断IP地址是否有效
        if ( preg_match( "/^([0-9]{1,3}.){3}[0-9]{1,3}$/", $userip ) == 0 ) {
            return 'IP Address Invalid';
        }
        //打开IP数据库
        if ( !$fd = @fopen( $dat_path, 'rb' ) ) {
            return 'IP data file not exists or access denied';
        }
        //explode函数分解IP地址，运算得出整数形结果
        $userip = explode( '.', $userip );
        $useripNum = $userip[0] * 16777216 + $userip[1] * 65536 + $userip[2] * 256 + $userip[3];
        //获取IP地址索引开始和结束位置
        $DataBegin = fread( $fd, 4 );
        $DataEnd = fread( $fd, 4 );
        $useripbegin = implode( '', unpack( 'L', $DataBegin ) );
        if ( $useripbegin < 0 )
            $useripbegin += pow( 2, 32 );
        $useripend = implode( '', unpack( 'L', $DataEnd ) );
        if ( $useripend < 0 )
            $useripend += pow( 2, 32 );
        $useripAllNum = ($useripend - $useripbegin) / 7 + 1;
        $BeginNum = 0;
        $EndNum = $useripAllNum;
        //使用二分查找法从索引记录中搜索匹配的IP地址记录
        while ( $userip1num > $useripNum || $userip2num < $useripNum ) {
            $Middle = intval( ($EndNum + $BeginNum) / 2 );
            //偏移指针到索引位置读取4个字节
            fseek( $fd, $useripbegin + 7 * $Middle );
            $useripData1 = fread( $fd, 4 );
            if ( strlen( $useripData1 ) < 4 ) {
                fclose( $fd );
                return 'File Error';
            }
            //提取出来的数据转换成长整形，如果数据是负数则加上2的32次幂
            $userip1num = implode( '', unpack( 'L', $useripData1 ) );
            if ( $userip1num < 0 )
                $userip1num += pow( 2, 32 );
            //提取的长整型数大于我们IP地址则修改结束位置进行下一次循环
            if ( $userip1num > $useripNum ) {
                $EndNum = $Middle;
                continue;
            }
            //取完上一个索引后取下一个索引
            $DataSeek = fread( $fd, 3 );
            if ( strlen( $DataSeek ) < 3 ) {
                fclose( $fd );
                return 'File Error';
            }
            $DataSeek = implode( '', unpack( 'L', $DataSeek . chr( 0 ) ) );
            fseek( $fd, $DataSeek );
            $useripData2 = fread( $fd, 4 );
            if ( strlen( $useripData2 ) < 4 ) {
                fclose( $fd );
                return 'File Error';
            }
            $userip2num = implode( '', unpack( 'L', $useripData2 ) );
            if ( $userip2num < 0 )
                $userip2num += pow( 2, 32 );
            //找不到IP地址对应城市
            if ( $userip2num < $useripNum ) {
                if ( $Middle == $BeginNum ) {
                    fclose( $fd );
                    return 'No Data';
                }
                $BeginNum = $Middle;
            }
        }
        $useripFlag = fread( $fd, 1 );
        if ( $useripFlag == chr( 1 ) ) {
            $useripSeek = fread( $fd, 3 );
            if ( strlen( $useripSeek ) < 3 ) {
                fclose( $fd );
                return 'System Error';
            }
            $useripSeek = implode( '', unpack( 'L', $useripSeek . chr( 0 ) ) );
            fseek( $fd, $useripSeek );
            $useripFlag = fread( $fd, 1 );
        }
        if ( $useripFlag == chr( 2 ) ) {
            $AddrSeek = fread( $fd, 3 );
            if ( strlen( $AddrSeek ) < 3 ) {
                fclose( $fd );
                return 'System Error';
            }
            $useripFlag = fread( $fd, 1 );
            if ( $useripFlag == chr( 2 ) ) {
                $AddrSeek2 = fread( $fd, 3 );
                if ( strlen( $AddrSeek2 ) < 3 ) {
                    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 ) )
                $useripAddr2 .= $char;
            $AddrSeek = implode( '', unpack( 'L', $AddrSeek . chr( 0 ) ) );
            fseek( $fd, $AddrSeek );
            while ( ($char = fread( $fd, 1 )) != chr( 0 ) )
                $useripAddr1 .= $char;
        } else {
            fseek( $fd, -1, SEEK_CUR );
            while ( ($char = fread( $fd, 1 )) != chr( 0 ) )
                $useripAddr1 .= $char;
            $useripFlag = fread( $fd, 1 );
            if ( $useripFlag == chr( 2 ) ) {
                $AddrSeek2 = fread( $fd, 3 );
                if ( strlen( $AddrSeek2 ) < 3 ) {
                    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 ) ) {
                $useripAddr2 .= $char;
            }
        }
        fclose( $fd );
        //返回IP地址对应的城市结果
        if ( preg_match( '/http/i', $useripAddr2 ) ) {
            $useripAddr2 = '';
        }
        $useripaddr = "$useripAddr1 $useripAddr2";
        $useripaddr = preg_replace( '/CZ88.Net/is', '', $useripaddr );
        $useripaddr = preg_replace( '/^s*/is', '', $useripaddr );
        $useripaddr = preg_replace( '/s*$/is', '', $useripaddr );
        if ( preg_match( '/http/i', $useripaddr ) || $useripaddr == '' ) {
            $useripaddr = 'No Data';
        } elseif ( !$this->is_utf8( $useripaddr ) ) {
            $useripaddr = iconv( 'GBK', 'UTF-8', $useripaddr );
        }
        return $useripaddr;
    }