License Page 8
Finally, it is very important to note that PHP4 is not distributed under the GNU GPL license as was PHP3. It is instead under a specific license called the PHP License. In essence, the PHP License is quite similar to the BSD license. It authorizes distribution and modification of PHP within open source or proprietary applications. Moreover, Zend, which is an integral part of PHP4, is distributed under yet another license, the Zend License, which was inspired by Troll Tech's QPL.
This license enables authors to take advantage of all of the contributions and corrections made by the community while reserving the right to make money off of the commercial licenses for the Zend library. However, this does not really concern PHP script programmers and designers.