A library is a collection of code used in your projects as is. This collection is presented in a file or a set of files stored in a directory (or in a number of directories).
Libraries may optionally include API documentation which enables you to use inline documentation extracted from the source code and to view the API documentation right from WebStorm.
Libraries are protected against any kind of modification, such as direct intentional editing or update result from refactoring. Libraries are also excluded from error checks. All this allows to save resource and improve performance and productivity.
All the PHAR files from the current project and the specified include path are shown in the project tool window under the Project View/Libraries/PHAR node and available for browsing right there. PHAR files are involved in indexing and therefore are used in autocompletion, search, reference resolution, etc.