Ideally, you should put a message in the header of the PHP file itself that explains the license. This way, the license declaration travels with the file.

I add this to the beginning of all my plugin's PHP files:

Copyright 2010 Samuel Wood (email : otto@ottodestruct.com)

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License version 2,
as published by the Free Software Foundation.

You may NOT assume that you can use any other version of the GPL.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

The license for this software can likely be found here:
http://www.gnu.org/licenses/gpl-2.0.html

Simple, easy.

You don't really have to include the entire license. Specifying which license is valid and giving probable location is enough. It's up to the user to find the terms and agree to them to gain the rights specified in the license.