How to get username from userid
 

I'm creating a php page for the forum that will show everybody from a specific string they input under the Additional Information / Details section. It's 'field2' located in the 'userfield' section of the forum database. Basically I am able to get the field2 string and userid, but I'm wondering how I can get the username from the userid?

Code:
if ($row['field2']=="United States") {
echo "" . $row['userid'] . " - " . $row['field2'] . "";
}
Thanks.