Code for e-mail tracking system

We need a simple mail tracking system (based on a tracking picture) which shall update our database with information about when the email was opened. We only send emails to our registered members and we are not involved in any form of spamming. Technical solution: In the end of the email we add some code (code1) which will include a unique number. When the receiver opens the email code1 access our server and get our logo back which shall be showed in the email. A program is implemented on our server which will update our database with help of the unique number. So what we need is the two pieces of code: Code to include in the emails and a PHP-program to implement on our server. It shall be made in a way to minimize risk of emails being blocked by firewalls and spam-filters. We will arrange the unique numbers and connection with our MySql-database etc. by ourselves.