Vanilla PHP provides you a lot of features to handle many use cases. However in order to not repeat yourself, you will soon need more libraries in your PHP application.
There are many places for you to find open source PHP libraries that you can use in your code. Today practically all PHP libraries from GitHub BitBucket, and GitLab can be found on http://packagist.org.
Packagist is a repository containing open source PHP packages, libraries, components, frameworks, even all-in-one software solutions such as CMSes, eCommerce and similar out-of-the-box working projects.
For using packagist you will need to use Composer, command line script for managing dependencies in your project.
Some PHP projects are that big that they have their own one-place locations for their specific extensions, bundles, or plugins - for open source and proprietary code.
Other useful resources to check out: