how to update date in a bridge table

[wxdqz]

In a bridge table, 4 keys:
userid, groupid, lastvisittime, totalvisit.
userid and groupid are foreign keys.

I can't update the lastvisittime and totalvisit.

My SQL is like:

$sql = "update usergroupline01 set lastvisittime='$now'where userid='$userid' and groupid='$groupid';";

Thanks for your help.