In a new post Brandon Savage suggests that, because of the openness of the PHP community and the hard work that's put into the language every day, those who aren't giving what they can back to the community might be "stealing" (in a sort of spirit of mutual giving kind of way).
That may sound harsh but consider: people who write open source software generally aren't paid to do it. There are a few exceptions (some people are paid to write Wordpress or the Zend Framework, for example). But the core of PHP is contributed by people who are solely interested in making PHP better for their professional endeavors. Not contributing to their efforts is like showing up at a potluck without bringing a dish. Not cool.
He also suggests a few ways that you can give back to the project and to the community like contributing to the documentation project, submitting a bug report, writing patches, etc.
The community exists because people choose to contribute. Giving back isn't a matter of charity, but a matter of the survival of the product or project you use every single day. It's not really optional, but it is easy.