As per OG:
I'm not a PHP guy, but it looks like you are trying to set the where of a numeric field using a string constant. I think your query should look something like:
$result = mysql_query("UPDATE users
SET username='$username',
dob='$dob',
location='$location',
group=$group
WHERE
id=$user_id");
Without the quotes on the $user_id (and I also removed them on $group since that looks numeric too).