Why can't the value of the SESSION variable be obtained? How to solve

Why can't the value of the SESSION variable be obtained? On the registration SESSION variable page, enter $ username & nbsp ;=& nbsp; $ HTTP_POST_VARS [& quot; username & quot;]; session_register (& quot; username & quot;); why can't the value of the SESSION variable be obtained?
The registration SESSION variable page is written as follows:
$ Username = $ HTTP_POST_VARS ["username"];
Session_register ("username ");

On another page, make the following judgment:
If (! Isset ($ _ SESSION ["username"]) | $ _ SESSION ["username"] = "")
{
Print ("username unregistered!
");
Print ("Login
");
Die ();
}
Else print ("username is registered!
");


$ HTTP_POST_VARS ["username"] already has a value, but what is printed on the judgment page is username unregistered !, Why?

PHP version 4.3.9. is it necessary to modify the PHP configuration file?

------ 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 your PHP configuration question?

I have also encountered one time before, that is, when the blank merchant recovers the system, my session will be unusable,

It is not completely unavailable, that is, the initial Shard value before the period is effective, and the cross-region operation will not work.

After that, he just needs to reset it once again ..