PHP License

The PHP License is the software license under which the PHP scripting language is released.

The PHP License is:

A non-copyleft free software license (according to the Free Software Foundation)
An open source license (according to the Open Source Initiative)

The license is GPL-incompatible due to restrictions on the usage of the term PHP.

Zend Engine License

While most of PHP is under the PHP License, most of the Zend Engine is under the similar Zend Engine License.