CRON Job Send email to users in DB? - php

Question

If I've got a database of users that have filled out a form, can I use a cron job to send an automated email? If so, what is the best way to "loop" it so that it sends the email once to each user?
$data = mysql_query("
SELECT *
FROM completed
WHERE
followupsent='0000-00-00 00:00:00'
AND valuesent + INTERVAL 4 DAY <= NOW()
")
or die(mysql_error());
while($info = mysql_fetch_array( $data ))
{
}
This checks to see if "followupsent" has been updated already as it updates with NOW() when it sends and also checks to see how many days since the value was sent.
I'm worried that by putting the email sending information in the while tags is going to loop for each row and end up sending a ton of emails.
Would using and if instead of a while:
if($info = mysql_fetch_array( $data ))
{
}
In order to send out to the first in the database and then let the cron job handle the rest by checking every minute which one is next?

Answer 1

Thats a perfectly fine way to do it. It will only send it it once per record (so assuming you have no duplicates in your completed table).
I assume you are updating the valuesent field in the loop

Answer 2

I suggest:
You ensure that your completed table uses a transactional storage engine, e.g. InnoDB:
ALTER TABLE completed ENGINE=InnoDB;
You define a new BOOLEAN column that indicates (to any other database connections) that a given record is in the process of being updated:
ALTER TABLE completed ADD COLUMN updating BOOLEAN NOT NULL DEFAULT FALSE;
You use a locking read to SELECT the records that are to be emailed, followed within the same transaction by an UPDATE to the newly created column. For example, using PDO:
$dbh->beginTransaction();
$select = $dbh->query('
SELECT *
FROM completed
WHERE followupsent IS NULL
AND valuesent <= CURRENT_TIMESTAMP - INTERVAL 4 DAY
AND NOT updating
FOR UPDATE
');
$dbh->exec('
UPDATE completed
SET updating = TRUE
WHERE followupsent IS NULL
AND valuesent <= CURRENT_TIMESTAMP - INTERVAL 4 DAY
AND NOT updating
');
$dbh->commit();
You can then update the database (removing the updating flag, together with whatever other status you require) as each email is sent:
$success = $dbh->prepare('
UPDATE completed
SET followupsent = CURRENT_TIMESTAMP,
updating = FALSE
WHERE id = ?
');
$failure = $dbh->prepare('
UPDATE completed
SET updating = FALSE
WHERE id = ?
');
foreach ($select as $row) {
$command = mail(...) ? $success : $failure;
$command->exec(array($row['id']));
}
Note that if the script terminates prematurely, the database may be left in an undesirable state—i.e. records may have updating=TRUE but there is no longer any script that is processing them; this could lead to some records not being emailed at all. You may want to guard against this eventuality by registering a custom shutdown function, or else by periodically inspecting the database for such "orphaned" records (however you must of course be sure that they are not currently being processed by a running script).