I ended up doing this:
// rewarding players who have been in the same gang every 50 days
$q = $db->query("
SELECT *
FROM `users`
WHERE `daysingang`%50=0
AND `daysingang`!=0
");
while($r = $db->fetch_row($q)){
event_add(
$r['userid'],
"You've been in your gang another 50 days! Congratulations. You've received $100,000 and 20 crystals.",
NULL
);
$db->query("
UPDATE `users`
SET `money`=`money`+100000, `crystals`=`crystals`+20
WHERE `userid`=" . $r['userid'] . "
");
}
:)