PHP get client IP and ip2long function

[php title="得到客户端IP地址"]
if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
$realip = $_SERVER['HTTP_X_FORWARDED_FOR'];
} elseif (isset($_SERVER['HTTP_CLIENT_IP'])) {
$realip = $_SERVER['HTTP_CLIENT_IP'];
} else {
$realip = $_SERVER['REMOTE_ADDR'];
}

$realip = ip2long($realip);
[/php]

ip2long 两个算法:

[php]
function ip2int($ip){
//我们先把ip分为四段,$ip1,$ip2,$ip3,$ip4
list($ip1,$ip2,$ip3,$ip4) = explode('.', $ip);
//然后第一段乘以256的三次方，第二段乘以256的平方，第三段乘以256
return $ip1*pow(256,3)+$ip2*pow(256,2)+$ip3*256+$ip4;
}
[/php]

[php title="用位运算"]
function ip2int($ip){
list($ip1,$ip2,$ip3,$ip4)=explode('.', $ip);
return($ip1>>24)|($ip2>>16)|($ip3>>8)|($ip4);
}
[/php]

有些ip转化成整数后，是负数，这是因为得到的结果是有符号整型，最大值是2147483647.要把它转化为无符号的，可以用sprintf("%u",ip2long($ip); 就能转换为正整数。而且得到的结果用long2ip可以正常转换回原来的ip地址。

ip2long也可以用于检测一个IP地址的有效性
int ip2long (string ip_address )

The function ip2long() generates an IPv4 Internet network address from its Internet standard format (dotted string) representation. If ip_address is invalid then -1 is returned. Note that -1 does not evaluate as FALSE in PHP.

注: As of PHP 5.0.0 ip2long() returns FALSE when ip_address is invalid