In PHP, how do I return an image byte by byte so I can test how long someone views an image?

I am wanting to track the amount of time an email is "read". This is from: http://stackoverflow.com/a/7754559

My question is specific to PHP and its server configuration. Here are my questions:

• How do you do this w/ php?


• Do I need any special server configs to do this? I use nginx, do I need to disable things like gzip?

Thanks!