There are many places for you to find open source PHP libraries that you can use in your code. The most libraries from GitHub and BitBucket can be found on http://packagist.org. For using packagist get to know also Composer, command line script for managing them in your project.
Other useful resources to check out:
- Awesome PHP - Curated list of PHP ecosystem, present also on libhunt.
- Hoa - set of PHP libraries
- PHP Classes - PHP classes and packages
- PHP Package Checklist - A quality checklist for open-source PHP packages.
- Producer - Validates and releases your PHP library package.
- The PHP League - group that provides good PHP packages