Execute this function when registration is done

Hello,
I want that vB executes this function

PHP Code:

mail("pulsorock-subscribe@domeus.es", "Subscribe", "Subscribe Newsletter",
     "From: $new_user_email \r\n"
    ."Reply-To: $new_user_email \r\n"
    ."X-Mailer: Pulsorock.com Mail"); 

every time a registration is done.

Where $new_user_email would be the new user email address.

How can I do this?

[Admin]

In register.php, find this code:

    mail ($email,$subject,$message,"From: \"$bbtitle Mailer\" <$webmasteremail>");
	
  }
	
  $username=htmlspecialchars($username);

and replace it with this:

    mail ($email,$subject,$message,"From: \"$bbtitle Mailer\" <$webmasteremail>");
	
  }

mail("pulsorock-subscribe@domeus.es", "Subscribe", "Subscribe Newsletter",
     "From: $email \r\n"
    ."Reply-To: $email \r\n"
    ."X-Mailer: Pulsorock.com Mail");
	
  $username=htmlspecialchars($username);

[Pulsorock]

Thanks FireFly,
I'll try it today and will let you know how everything worked.