Subdirectory Creation upon Registration

[defi]

Okay, basically, what I'm trying to do is autocreate a subdirectory of the user's registered username which is created after activation on vbulletin 3.0.7;

In register, I placed this code:

//make directory
            mkdir($username, 0777);

in between:

// activate account
			$username = unhtmlspecialchars($userinfo['username']);

and,

if ($vboptions['welcomemail'] AND !$userinfo['posts'])
			{
				eval(fetch_email_phrases('welcomemail'));
				vbmail($userinfo['email'], $subject, $message);
			}

The problem is that it doesn't create the directory at all - no errors even pop-up when the account is registered and confirmed???

Does anyone have any ideas or suggestions?