echo "selecting userids for 68104<br>";

$sql = 'SELECT userid FROM `phplist_user_user_attribute` WHERE `attributeid`= 7 AND `value` LIKE "68104"';
$query = mysql_query($sql);

echo mysql_num_rows($query), "rows selected<br>";

if (mysql_num_rows($query)) {
$ary = implode(',', mysql_fetch_assoc($query));
echo $ary, "<br>";
$update = "UPDATE  `phplist_user_user_attribute`  SET  `value`  =  'on' WHERE  `attributeid`  ='16' AND  `userid` IN (".$ary.")";
mysql_query($update);
} 

selecting userids for 68104
23rows selected
userid: 21