Fix menu inconsistencies - Chapter 7

_posts/07-05-01-Abstraction-Layers.md

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+---
+isChild: true
+title:   Abstraction Layers
+anchor:  databases_abstraction_layers
+---
+
+## Abstraction Layers {#databases_abstraction_layers_title}
+
+Many frameworks provide their own abstraction layer which may or may not sit on top of [PDO][1]. These will often
+emulate features for one database system that is missing from another by wrapping your queries in PHP methods, giving
+you actual database abstraction instead of just the connection abstraction that PDO provides. This will of course add a
+little overhead, but if you are building a portable application that needs to work with MySQL, PostgreSQL and SQLite
+then a little overhead will be worth it the sake of code cleanliness.
+
+Some abstraction layers have been built using the [PSR-0][psr0] or [PSR-4][psr4] namespace standards so can be
+installed in any application you like:
+
+* [Aura SQL][6]
+* [Doctrine2 DBAL][2]
+* [Propel][7]
+* [ZF2 Db][4]
+
+
+[1]: http://php.net/book.pdo
+[2]: http://www.doctrine-project.org/projects/dbal.html
+[4]: http://packages.zendframework.com/docs/latest/manual/en/index.html#zend-db
+[6]: https://github.com/auraphp/Aura.Sql
+[7]: http://propelorm.org/
+[psr0]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md
+[psr4]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader.md