http://www.reddit.com/r/PHP/comments/a9mio/lets_compile_a_li...
Symfony is one of them and has some of the best PHP code out there:
https://github.com/symfony/symfony/
As well as Aura: https://github.com/auraphp
And Laravel: https://github.com/laravel/laravel
Particularly, the newer PHP frameworks, Guzzle, Composer, and PHPUnit are good to look at.
It should be no surprise that structured and organized PHP is better than raw PHP, and that examples of well written PHP would probably involve a framework.
Although I suppose it could be argued that since PHP is intended for the web, it shouldn't need a framework, and it should support more of the features a framework provides out of the box than it does. Nevertheless, the pattern of a framework is as useful in PHP for the same reasons as it is elsewhere, so I don't believe it says anything negative about the language per se.
They are very high-quality libraries.