Calling PHP through an image?

I've seen this more and more lately. Whether it be PNG, or JPEGS people are calling what looks like to be a PHP file or some other type of script through an image.

For example something like this

<a href="http://www.example.com/" target="_blank">
<embed src="http://www.example.com/x/sample.jpg" border="0" height="0" width="0">
</a> 

PHP:

How does something like this work?

 

Here you don't call any php code...

 

do you mean them images that show info on but are images?

like the ones that say your ip etc etc

 

Here you are :

     header("Accept-Ranges: bytes");
     header("Cache-Control:");
     header("Pragma:");
     header("Content-Length: $len");
     header("Connection: close");
     header("Content-Type: image/png");
     header("Content-Disposition: inline; Filename=image.png");
     readfile($IMAGE);

PHP:

 

Does the image have to be png?

 

You can use whatever you like.

Just change the headers and the filename (actually the filename is not neccesery but it may be needed by some browsers)

 

Thanks this is what I figured.

For the people that didn't clearly get what I asked I'll try and explain it again...sorry for the mess up there.

Basically fooling the page into thinking you are calling an image file when it really is running a PHP script. It's something I see being done on alot of trackers...and just wondered how it was done.