Making PHP's mail() asynchronous

You have a lot of ways to do this, but handling thread is not necessarily the right choice.

• register_shutdown_function: the shutdown function is called after the response is sent. It's not really asynchronous, but at least it won't slow down your request. Regarding the implementation, see the example.
• Swift pool: using symfony, you can easily use the spool.
• Queue: register the mails to be sent in a queue system (could be done with RabbitMQ, MySQL, redis or anything), then run a cron that consume the queue. Could be done with something as simple as a MySQL table with fields like from, to, message, sent (boolean set to true when you have sent the email).

Example with register_shutdown_function

<?php
class MailSpool
{
  public static $mails = [];

  public static function addMail($subject, $to, $message)
  {
    self::$mails[] = [ 'subject' => $subject, 'to' => $to, 'message' => $message ];
  }

  public static function send() 
  {
    foreach(self::$mails as $mail) {
      mail($mail['to'], $mail['subject'], $mail['message']);
    }
  }
}

//In your script you can call anywhere
MailSpool::addMail('Hello', '[email protected]', 'Hello from the spool');


register_shutdown_function('MailSpool::send');

exit(); // You need to call this to send the response immediately

php-fpm

You must run php-fpm for fastcgi_finish_request to be available.

echo "I get output instantly";
fastcgi_finish_request(); // Close and flush the connection.
sleep(10); // For illustrative purposes. Delete me.
mail("[email protected]", "lol", "Hi");

It's pretty easy queuing up any arbitrary code to processed after finishing the request to the user:

$post_processing = [];
/* your code */
$email = "[email protected]";
$subject = "lol";
$message = "Hi";

$post_processing[] = function() use ($email, $subject, $message) {
  mail($email, $subject, $message);
};

echo "Stuff is going to happen.";

/* end */

fastcgi_finish_request();

foreach($post_processing as $function) {
  $function();
}

Hipster background worker

Instantly time-out a curl and let the new request deal with it. I was doing this on shared hosts before it was cool. (it's never cool)

if(!empty($_POST)) {
  sleep(10);
  mail($_POST['email'], $_POST['subject'], $_POST['message']);
  exit(); // Stop so we don't self DDOS.
}

$ch = curl_init("http://" . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);

curl_setopt($ch, CURLOPT_TIMEOUT, 1);
curl_setopt($ch, CURLOPT_NOSIGNAL, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, [
  'email' => '[email protected]',
  'subject' => 'foo',
  'message' => 'bar'
]);

curl_exec($ch);
curl_close($ch);

echo "Expect an email in 10 seconds.";

Use AWS SES with PHPMailer.

This way is very fast (hundreds of messages per second), and there isn't much code required.

$mail = new PHPMailer;
$mail->isSMTP();                                      // Set mailer to use SMTP
$mail->Host = 'ssl://email-smtp.us-west-2.amazonaws.com';  // Specify main and backup SMTP servers

$mail->SMTPAuth = true;                               // Enable SMTP authentication

$mail->Username = 'blah';                 // SMTP username
$mail->Password = 'blahblah';                           // SMTP password


$mail->SMTPSecure = 'tls';                            // Enable TLS encryption, `ssl` also accepted
$mail->Port = 443; 

Not sure if i interpreted your question correctly but i hope this helps.