keeping track of external image linking with php?

I don't think that's the best subject, but I'm not really sure how to phrase it. I'm not familiar with php's image capabilities. I've read about them a bit, but never really used them. I'd like to have an image that when accessed would cause php to kick in and do a few things before sending the image along to whoever is requesting it. Basically I'd like to keep a running tally of how many times a website is accessing an image on my server. Which functionally speaking is identical to a hit counter I guess. Is it possible to have on a webpage something like <img src="image.php?website=awebsite.com"> and then image.php takes note of the website variable, does a few things with mysql, then passes on the image that is wanted, utilizing something from the gd library? Thanks, Matt