P3P Explained: Compact Privacy Policies in 2010

Your first point of reference for P3P should be the wikipedia page. It'll give you a good overview. Here's the basics:

• Microsoft helped pay for it.
• Microsoft browsers are the only browsers that still use it.
• Microsoft browsers have never used it correctly.

When Internet Explorer (IE6 - IE8) is in the "High" Privacy mode it "Blocks all cookies from websites that do not have a compact privacy policy." So if you want to set a cookie on an IE browser in "High" Privacy, you need a compact privacy policy.  You don't need a well formed policy, you don't need a valid policy, you can put swears in your policy and it'll still be accepted.

In PHP:
<?php header('P3P: CP="JUNK DATA"'); >