[Xenon]

yes it'll only work for people which register after you uploaded the new file.

as satan said, you could run an sql-query to use it for already registred users:
[sql]
UPDATE user SET homepage='http://url' WHERE homepage=''
[/sql]

and to deny users to edit the homepage to a blank, open member.php find

PHP Code:

$DB_site->query("UPDATE user SET birthday='".addslashes($birthday)."',signature='".addslashes($signature)."',customtitle='".intval($customtitle)."',usertitle='".addslashes($customtext)."',email='".addslashes(htmlspecialchars($email))."',parentemail='".addslashes(htmlspecialchars($parentemail))."',coppauser='$coppauser',homepage='".addslashes(htmlspecialchars($homepage))."',icq='".addslashes(htmlspecialchars($icq))."',aim='".addslashes(htmlspecialchars($aim))."',yahoo='".addslashes(htmlspecialchars($yahoo))."',usergroupid='$bbuserinfo[usergroupid]' WHERE userid='$bbuserinfo[userid]'"); 


and there also add before:

PHP Code:

if(trim($homepage)=="") $homepage="http://url";