The documentation is within PIE.php itself
This file is a wrapper, for use in PHP environments, which serves PIE.htc using the
correct content-type, so that IE will recognize it as a behavior. Simply specify the
behavior property to fetch this .php file instead of the .htc directly:
[ ...css3 properties... ]
This is only necessary when the web server is not configured to serve .htc files with
the text/x-component content-type, and cannot easily be configured to do so (as is the
case with some shared hosting providers).