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Propel is an open-source Object-Relational Mapping (ORM) for PHP which implements the ActiveRecord pattern. It allows you to access your database using a set of objects, providing a simple API for storing and retrieving data. Propel uses PDO as an abstraction layer and code generation to remove the burden of runtime introspection.
A few years ago, Propel was a very popular alternative to Doctrine. However, its popularity has rapidly declined and that's why the Symfony book no longer includes the Propel documentation. Read the official PropelBundle documentation to learn how to integrate Propel into your Symfony projects.