License compatibility is a very hot subject and the Open Source Initiative (OSI) has issued a Statement on License Proliferation and its impact on code sharing & reuse.
After careful analysis and many discussions, it has been concluded that it is appropriate to use code (such as libraries) licensed PHP, BSD,
Apache, ( Apache 2.0 is only compatible with LGPL v3 - see http://www.dwheeler.com/essays/floss-license-slide.html ) MIT and LGPL (except for incorporating code licensed only under the LGPL v3; using it as a library is fine) within Tiki. We can not however use GPL code, unless it is dual-licensed with one of the 5 above.
These 5 licenses are precisely the licenses the PEAR group recommends in their announcement where they state "[you] should fairly safely be able to also use any PEAR package without licensing worries, be it for commercial or non commercial, closed or opensource use."
Dual-licensing was another issue. Here, we agree with the fine folks at Mozilla:
If a piece of code is dual- or triple-licensed, that means that someone (the licensee) who modifies and/or distributes it can choose which of the available sets of license terms they are operating under. Therefore, we can continue to use libraries such as phplot which is dual licensed PHP and GPL.
The Tiki team strives to respect the wishes of authors and to respect all licenses. If you are aware of any code in Tiki which you think is inappropriately licensed for the project, please bring it to the attention of the developers on the dev list or IRC channel.
For the Tiki community,
marclaporte @ php DOT net
2010-12-20 note: zlib License is also OK.
The rest of the page is kept as background information. Thank you to everyone who contributed.