Why is the value of the session variable not being taken? How to fix it

Why is the value of the session variable not being taken?
The page that registers the session variable is written like this:
$username = $HTTP _post_vars["username"];
Session_register ("username");

On another page, judge this:
if (!isset ($_session["username") | | $_session["username"]== "")
{
Print ("Username unregistered!
");
Print ("Login
");
Die ();
}
else print ("username is registered!
");


$HTTP _post_vars["username"] already has the value, but in the judgment of the page printed out is username unregistered!, this is why?

PHP version is 4.3.9, is not the PHP configuration file to do what changes?

------Solution--------------------
Session_Start ();
$username = $HTTP _post_vars["username"];
$_session[' username '] = $username;
------Solution--------------------
Session_Start ()?
------Solution--------------------
$username = $HTTP _post_vars["username"];
Session_register ("username");

==>
$_session["username"]= $username;
------Solution--------------------
is not your PHP setup problem?

I also encountered the previous one, is the space business to restore a system, my session can not afford to use,

Nor is it completely useless, that is, when the front page is valid, the page is not available.

And then he set it up again.

