License questions

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

Neither the name of Yii Software LLC nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

What does the marked text mean?

I’m not a lawyer but I think it means that even if you compile the PHP code into binary (using phc, Phalanger or whatever) you must reproduce the copyright notice and specifically never mention anyone related to Yii’s development as endorsing your application without permission.

That means you can use Yii for both commercial and open source products with almost no restrictions.

The only thing you can’t do without asking Yii authors first and getting their permission is to officially tell people that your project that uses Yii or is a fork of Yii is good because it’s based on Yii.