Emailing people when there is a new comment - php

Question

I'm extremely new to PHP and my first project was developing a comment system. So now it is great but I would like to give the commenters the choice to get emails when there is a new comment. First I decide to create another row in my table called:
'email_notifications'. In the submit.php for the comment form.
To email everyone.... I wan't to use:
$emails = mysql_query("SELECT * FROM email_notifications");
while($row=mysql_fetch_assoc($emails)) {
mail($row['email'],'New Comment on...','There was a new comment o....',"From: no-reply#jdrag.x10.mx");
}
I'm not even sure this would work. But my first concern was that it was going to email people more than once.
Because, if people comment more than once with the same email, wont it just email that email more than once when it goes though the database for all the emails. Thats my big concern. Anyway I could email people when there is a new comment without email people more than once each time there is a new comment.
Summary(what I want to accomplish):
1. Someone submits a comment.
2. everyone above gets an email
I hope I made myself clear. If not, I apologize. Thanks in advance. Remember, im still a beginner :)

Answer 1

Assuming your table contains everyones email address this should work:
$emails = mysql_query("SELECT * FROM email_notifications");
while($row=mysql_fetch_assoc($emails)) {
$subject = "There was a new comment";
$message = "no-reply#jdrag.x10.mx";
mail($row['email'],$subject,$message);
}
However I would try to use Prepared Statments or Stored Proceedures for your database calls.
Heres the same example using Prepared Statements:
$mysqli=new mysqli("server", "username", "password", "database");
$stmt = $mysqli->prepare("SELECT email FROM email_notifications");
$stmt->execute();
$stmt->bind_result($email);
while($stmt->fetch()){
$subject = "There was a new comment";
$message = "no-reply#jdrag.x10.mx";
mail($email,$subject,$message);
}

Answer 2

I would not recomment putting the mail() function inside any loop, there's a possibility to create too many mails in a short timestan, which will probably result in your domain considered a spammer.
In your code where the new comment is created you can track all distinct identifiers for all comments before the created one:
$query = mysql_query("SELECT DISTINCT `userId` FROM `comments` WHERE `object` = {$yourObject} AND `userId` != {$currentUser}");
This will select all user identifiers except for the user who posted the latest comment. Then you have to create the notifications in your appropriate table, watching out for duplicates so as to prevent multiple identical notifications.
Last part is creating a mail queue script on your server that will slowly send the notifications. There are many ways to to that, i prefer a CLI script which is launched via crontab (if not launched already). There, there's just an infinite-looping cycle with a sleep of like 5 seconds to a minute (if no notifications are found) and a simple mail() sending. Here's an improvised example based on an existing CLI script:
#!/usr/bin/php -q
<?php
// acquire a process id file or prevent launching a second instance.
$filename = "/path/to/mailer.pid";
$file = fopen($filename, 'w+');
if (!flock($file, LOCK_EX + LOCK_NB)) exit(0);
// force an uninterruptable script
ignore_user_abort(true);
set_time_limit(0);
// mysql connection routines somewhere here
while (true) {
$query = mysql_query("SELECT * FROM `mail_notifications` ORDER BY `timestamp` ASC LIMIT 0, 1");
if (!mysql_num_rows($query)) {
// Oh, no notifications pending. Take a little break
sleep(60);
continue;
}
$data = mysql_fetch_assoc($query);
// your mailing magic here
sleep(5);
}
?>
Serious people write POSIX-compatible scripts which can be launched as daemons, I didn't have the urge to try that, though. The infinite loop is a dangerous thing, but that works for me at least.