Get source information from dynamic "image"

I am currently developing a newsletter marketing system, to which I want to开发者_开发技巧 include certain statistics. I'm currently capable of keeping track of the amount of views a certain mailing has gotten by including a php file as an image that updates the database.

<img src="http://siteroot/counter.php" />

Now I am wondering if it is possible to retrieve any information such as OS or browser (probably not) and if not, if it is possible to retrieve the url on which the e-mail was read.

I want to know this so I can see what kind of e-mail client was used to read the e-mail and things like that.

You can access $_SERVER['HTTP_USER_AGENT'] in your script, which generally contains the browser and OS version information, and occasionally what toolbars/extensions/plugins are installed. It's not 100% reliable, as the information is under user control.