Doctrine is a collection of projects built for PHP. Each project can be used standalone and installed with Composer.
PHP Doctrine DocBlock Annotations Parser library that lets you parse structured data out of PHP docblocks for use with other functionality.
PHP Doctrine Cache library is a popular cache implementation that supports many different drivers such as redis, memcache, apc, mongodb and others.
PHP Doctrine Collections library that adds additional functionality on top of PHP arrays.
PHP Doctrine Common project is a library that provides additional functionality that other Doctrine projects depend on such as better reflection support, persistence interfaces, proxies, event system and much more.
Powerful PHP database abstraction layer (DBAL) with many features for database schema introspection, schema management and PDO abstraction.
PHP Doctrine Inflector is a small library that can perform string manipulations with regard to upper/lowercase and singular/plural forms of words.
PHP Doctrine Migrations project offer additional functionality on top of the database abstraction layer (DBAL) for versioning your database schema and easily deploying changes to it. It is a very easy to use and a powerful tool.
PHP Doctrine MongoDB project is a library that provides a wrapper around the native PHP Mongo PECL extension to provide additional functionality.
PHP Doctrine MongoDB Object Document Mapper (ODM) provides transparent persistence for PHP objects to MongoDB.
PHP object relational mapper (ORM) that sits on top of a powerful database abstraction layer (DBAL). One of its key features is the option to write database queries in a proprietary object oriented SQL dialect called Doctrine Query Language (DQL). This provides developers with a powerful alternative to SQL that maintains flexibility without requiring unnecessary code duplication.
These were once active and maintained projects. If you are interested in helping to maintain any Doctrine project take a look at the open issues on GitHub and submit pull requests.