Which php framework or CMS code is suitable for beginners to read and learn?

Which php framework or CMS code is suitable for beginners to read and learn? Younger brother has one year of php experience. he has been developing small website outsourcing for a single person. he has been developing dede and discuz twice, and has used only thinkphp for the framework. he has never carefully read their code.
Which mature php framework or CMS product code is suitable for beginners who have a little foundation like me?

Reply to discussion (solution)

Thinkphp is simple enough. there are more people communicating with each other.

Thinkphp is simple enough. there are more people communicating with each other.

Simplicity is relative...

TP .. Currently, it is widely used in China.

Thinkphp is simple enough. there are more people communicating with each other.

TP .. Currently, it is widely used in China.

It is said that the TP coupling is too high, and MVC is chaotic, and there is almost no design pattern, which is much worse than CI and Yii.
However, it is quite popular in China. you should take a look at the commonly used words.
I mainly want to learn about program architecture and design patterns.

Then you can look at zend, yii, or ci ,? Some frameworks are famous.

Then you can look at zend, yii, or ci ,? Some frameworks are famous.

The next two are relatively lightweight and many people have mentioned them. they are a good idea.
Zend...