Track User After Success Login

Hi.
My Register/Login Script is in Php.
How to track users ?
for example after Success Login we can track user "username" and "userid" with sessions ? how to register variables into a session ?

some part of my codes

login proccess

if ($user==$dbuser && $pass=="$dbpass" ){

session_register("loginuser");

PHP:

Success Login - Content Page

<? 

session_start();

if(session_is_registered("loginuser")){

}
else {
die ("access denied") ;
}

PHP:

for example if "smith" user login to system we have a varible to show system current user is "smith".