php如何获取客户端的IP？

<?php
function _get_client_ip() {
	$ip = $_SERVER['REMOTE_ADDR'];
	if (isset($_SERVER['HTTP_CLIENT_IP']) && preg_match('/^([0-9]{1,3}\.){3}[0-9]{1,3}$/', $_SERVER['HTTP_CLIENT_IP'])) {
		$ip = $_SERVER['HTTP_CLIENT_IP'];
	} elseif(isset($_SERVER['HTTP_X_FORWARDED_FOR']) AND preg_match_all('#\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}#s', $_SERVER['HTTP_X_FORWARDED_FOR'], $matches)) {
		foreach ($matches[0] AS $xip) {
			if (!preg_match('#^(10|172\.16|192\.168)\.#', $xip)) {
				$ip = $xip;
				break;
			}
		}
	}
	return $ip;
}
$ip=_get_client_ip();
echo $ip;
?>