run it via a cron that will check what is says and then set it in the user table.
i.e.
PHP Code:
$rawdata = $DB_site->query("
SELECT userid, fieldX
FROM usertextfield
WHERE LIKE '" . strtolower('%yes%') . "'
");
while ($users_to_change = $DB_site->fetch_array($rawdata))
{
$useridstochange .= " $users_to_change[userid]";
}
$newuserid = trim($useridstochange);
$DB_site->query("
UPDATE users
SET receiveadminemails = 1 ## or whatever the value is ##
WHERE userid IN($newuserid)
");
something a little like that