<?php

function getClientIP()

{

    if (getenv("HTTP_CLIENT_IP"))

        $ip = getenv("HTTP_CLIENT_IP");

    else if(getenv("HTTP_X_FORWARDED_FOR"))

        $ip = getenv("HTTP_X_FORWARDED_FOR");

    else if(getenv("REMOTE_ADDR"))

        $ip = getenv("REMOTE_ADDR");

	else $ip = "Unknow";

    return $ip;

}

session_start();

$_SESSION['id'] = getClientIP();	// 将客户端ip传入session, 在c.php中取

$sessionid = $_SESSION['id'];

var_dump($sessionid);

<?php

session_start();

$clientIp = $_SESSION['id'];	// 取session出来用

var_dump($clientIp);