What is the best way for a php to call an external php file? I am using cakephp. I want to send out 2 or 3 emails after user registered. However, it takes a long time to send out the email. I want to spin another process to send email and return the UI right away.

What is the best way to do it? Do a system call out? Or use message queue? Or cron job?

Well you don't need to 'notify' another php file. Just add in the mail() function to whatever php script processes the form when a user registers

OR

If you mean send an email out to everyone who's registered you just use the mail() function in the form process.

I might not be interpreting what you mean correctly. If you could explain a bit more that would be helpful.

PHP Code:

# This is an example, it may/may not work for your situation.
# Please read this code and get an idea of what to do.
if ($userReged == true) { // If the user has registered
$userEmail = "example@domain.com";
$msgType = 'newUser';
     email_user($userEmail,$msgType); // Call the function
}
function email_user($userEmail,$msgType) {
  if ($msgType == 'newUser') { // If the email needs to be about a new user.
    $msg = "Thanks for registering, your password has been encrypted into our database and can not be unencrypted.";
  }
     mail($userEmail,"Congrats on Registering",$msg,$headers);
flush(); // Ensure that the email has sent.
header("Location: thanks.html"); // Redirect
} 


Something along those lines.

*Edit* Agh, peachy beat me */EDIT*