Let's say you assign each subscriber in your database a number. You send a mailing out to the list. In the mail to the first subscriber, you include this tag:

Let's say you assign each subscriber in your database a number. You send a mailing out to the list. In the mail to the first subscriber, you include this tag: <img src="http://www.example.com/track.php?id=1"> http://www.example.com/track.php?id=1"> In the mail to the second subscriber, you include this tag: <img src="http://www.example.com/track.php?id=2"> http://www.example.com/track.php?id=2"> And so on. There needs only be one file on your server to track opens no matter how many people you mailed: track.php. That one file reads the "id" parameter from the request to know which subscriber opened the mail, and logs that in your database. It then creates a 1x1 transparent image, in memory, using GD, and outputs that to the browser that loaded the image tag. The image never existed on disk, and nothing needed to be created in advance of sending a mail.