Redirect url and add new ID every time

Hi,

I am a beginner as trying to get a phpfile to redirect to a another url, but also to change a part of the url adress in the php every time its clicked on.
So on the website its a textlink <a href="www.playnow.php">PLAY NOW</a> and the www.playnow.php should redirect the user to the downwards url + changing a part of the url (XXXXXXXXXXXXXXXX in the link down wards).
In this I want the XXXXXXXXXXXXXXXX to be a number as autochange the number everytime someone click <a href="www.playnow.php">PLAY NOW</a> so the number never are the same. The number can be short or long.

The link adress in the playnow.php is:
http://gv1.rmd.com/playVideo?siteId...nst_/smil:Australia_iphone.smil/playlist.m2u1

So its the XXXXXXXXXXXXXXXX as need to be a number and autochange.

Anyone as know how to do this?