根据IP获取访客所在国家/城市/经纬度
安装GeoIP扩展：

sudo apt-get install libgeoip-dev

pecl install geoip-1.1.0 

注意:Beta版要指定版本号.如果是apt安装的PHP,直接安装php5-geoip这个包即可.
php.ini中加入:

extension=geoip.so
geoip.custom_directory="/usr/share/GeoIP"

免费下载GeoLiteCity数据库(解压后18MB):
http://dev.maxmind.com/geoip/legacy/install/city/

wget http://geolite.maxmind.com/download/geoip/database/GeoLiteCity.dat.gz
gunzip GeoLiteCity.dat.gz
sudo mkdir -v /usr/share/GeoIP
sudo mv -v GeoLiteCity.dat /usr/share/GeoIP/GeoIPCity.dat

测试:

php -a

<?php
print_r(geoip_record_by_name("106.37.165.80")); //回车后按Ctrl+D运行
Array
(
 [continent_code] => AS
 [country_code] => CN
 [country_code3] => CHN
 [country_name] => China //国家
 [region] => 22
 [city] => Beijing //城市
 [postal_code] =>
 [latitude] => 39.928901672363 //纬度
 [longitude] => 116.38829803467 //经度
 [dma_code] => 0
 [area_code] => 0
)

在命令行用geoiplookup查看IP信息:

traceroute www.oschina.net 

可见IP地址

 61.145.122.155

sudo apt-get install geoip-bin geoip-database
geoiplookup 61.145.122.155 -f /usr/share/GeoIP/GeoIP.dat
GeoIP Country Edition: CN, China

geoip-database提供的GeoIP.dat只能精确到国家.

geoiplookup 61.145.122.155 -f /usr/share/GeoIP/GeoIPCity.dat
GeoIP City Edition, Rev 1: CN, 30, Guangdong, Guangzhou, N/A, 23.116699, 113.250000, 0, 0

从maxmind官网下的数据库GeoLiteCity则信息更详细.

geoiplookup 61.145.122.155 则同时显示上述两个数据库的信息.

根据IP确定经纬度与计算距离

可以用

geoip_record_by_name($_SERVER["REMOTE_ADDR"])

根据用户IP确定经纬度.
注意:

geoip_record_by_name()

返回的西经和南纬是负数.

5000米转成经纬度:
纬度 Latitude:  1 deg = 110852 m
经度 Longitude: 1 deg = 111320*cos(lat) m
同一经线上,相差一纬度约为 110852 米
同一纬线上,相差一经度约为 111320*cos(lat) 米 (lat为该纬线的纬度)

<?php
//以当前用户经纬度为中心,查询5000米内的其他用户
$y = 5000 / 110852; //纬度的范围
$x = 5000 / (111320*cos($lat)); //经度的范围
$sql = "
 select * from user where 
 lat >= ($lat-$y) and lat <= ($lat+$y) and 
 lon >= ($lon-$x) and lon <= ($lon+$x);
";

数据库用户表中设两个字段,分别存储用户的经度lat和纬度lon.

($lat-$y) <= lat <= ($lat+$y)
($lon-$x) <= lon <= ($lon+$x)

这个范围是一个粗略的范围,下面计算距离后把超过5公里的用户去掉即可.

根据上面查询出来的用户的经纬度,
用半正矢公式(Haversine)根据经纬度计算两点间距离:

<?php
function distance($lat1, $lon1, $lat2, $lon2) {
 $R = 6371393; //地球平均半径,单位米
 $dlat = deg2rad($lat2-$lat1);
 $dlon = deg2rad($lon2-$lon1);
 $a = pow(sin($dlat/2), 2) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * pow(sin($dlon/2), 2);
 $c = 2 * atan2(sqrt($a), sqrt(1-$a));
 $d = $R * $c;
 return round($d);
}
echo distance(0, 0, -1, 0); // 111202米

然后就可以用uasort或array_multisort由近到远列出用户了,比如有名为win,osx,lin这3个用户:

<?php
$arr = array(
 "win" => array(
  "dis" => 1024,
  "age" => 31
 ),
 "osx" => array(
  "dis" => 512,
  "age" => 15
 ),
 "lin" => array(
  "dis" => 512,
  "age" => 25
 )
);
foreach($arr as $k => $v) {
 $sort["dis"][$k] = $v["dis"];
 $sort["age"][$k] = $v["age"];
}
//先按距离升序排序,如果距离相同,则按年龄降序排序
array_multisort($sort["dis"], SORT_ASC, $sort["age"], SORT_DESC, $arr);
echo json_encode($arr);
//{"lin":{"dis":512,"age":25},"osx":{"dis":512,"age":15},"win":{"dis":1024,"age":31}}