PHP Persistence. Concepts, Techniques and Practical Solutions with Doctrine. M. Romer

 PHP developers nowadays generally think and code in an object-oriented way.
 Application functionality is built using classes, objects, methods, inheritance, and other object-oriented techniques. In the beginning, OOP was used primarily for the general technical aspects of applications, such as MVC frameworks or logging and mailing libraries. All these components can be used more or less “as is” in other applications, regardless of the domains those applications inhabit: for example, e-commerce, portal, or community site. For complex systems, or if aspects such as maintainability and extensibility are important, OOP is also an advantage in the domain-specific code. Basically, every application consists of two types of code: general technical code and domain-specific code. General technical code is often reusable when built as a library or framework; domain-specific code is often too customized to be reused.