How to fetch a userid

[Sprite-]

I am writing a script that uses vbulletin for usernames and userids.

Currently, I know I can use

$info = fetch_userinfo($userid);
$username=$info["username"];
echo $username;

to display a username for a userid.

My problem is going the other way around. Does anyone know how to display a userid for a username?

Any help would be appreciated. Thanks!

Edit: Also I am aware that I can do this with an sql query. I don't want to have to do that.