Federico Cargnelutti has written up a brief look at PHP's contribution to Open Source. More specifically, how its helped the online community save money and have better software for it.
Open source developers don't write FLOSS applications because they have to, they write them because they want to. Motivation is not always driven by money, most of the time PHP developers do it for the joy of it.
He lists some of the motivation factors behind the work that a lot of PHP (and Open Source) developers do for and in the community like the potential to learn, credibility/notoriety, sense of purpose and the possibility of a financial reward. He also includes just a few of the Open Source projects PHP developers have contributed to the community such as WordPress, Magento and Gallery.