On en parle beaucoup depuis quelques semaine : la licence de PHP4 (à la différence de PHP3) est incompatible avec la GNU GPL.

Le site GNU précise :

The PHP License, Version 2.02 (http://www.php.net/license/2_02.txt)

This is the license used by PHP4.

This is a non-copyleft free software license with practical problems like those of the original BSD license, including incompatibility with the GNU GPL.

PHP3 is not under this license. PHP3 is disjunctively dual-licensed with the GNU GPL. Thus, while PHP4 (which is covered only by the PHP 2.02 License) is still free software, we encourage you to use and make improvements to only PHP3. That way, we can have an active version of PHP whose license is compatible with the GPL. If you are interested in helping maintain an active version of PHP3, please contact the GNU Volunteer Coordinators .

Consulter le lien