php session variables

I'm working with sessions for the first time and why does the variable $session_life always = 0?

$_SESSION['time']开发者_如何学Python = time();
$inactive = 30;
$session_life = time() - $_SESSION['time'];

time() returns the current timestamp in seconds - less than a second has passed between the first and third lines. Try using microtime() instead:

http://us2.php.net/manual/en/function.microtime.php

If those expressions happen one after the other what you're doing is essentially:

time() - time();

which is likely to return 0.