reporting opened emails

I know that mail open statistics do not work in gmail client because images are turned off by default

but how does sendgrid, constant contact
report that mail is open

maybe they use an image
but how does the image relate to an id which is unique
so I can tell which user opened email

could you answer in php
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mwochnickConnect With a Mentor Commented:
Not sure what you mean by responding in PHP, but generally what happens with products like  constant contact is that they put something in the html of the email (like the image you suggest) the causes a GET on the server.  Usually they will create a URL to that image contains  a unique string that is tied to the user's email address, the campaign being run etc.

The web server can either have a rule to respond to all GETs that match the pattern with the image and log a message in the web server logs.  Then all you need to do is parse the log files for the unique IDs and any records that match the key tied to email address indicates an open.

Alternatively a web application could be written that responds to that same pattern and does the logic (parsing the URL request for the token) and logging that information to a database or whatever you desire.

To build out that functionality in PHP you would need to
- Create a program that generates the unique string for each email address and campaign combo - like an MD5 hash
- Create a program to generate and send the emails with the unique id in the email body
- Create the web application or web server rule to respond to the image
rgb192Author Commented:
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