It may seem like an unusual recommendation but learning the fundamentals and process of OOP design will best serve you learning it.
To this end, Behat ( behat.org ) is a tool that will let you express in words what you want to do and will write skelton classes for you. It will stop you from focusing how to code and more about what you want to do.
I would then look to learning TDD ( Test Driven Development ) using PHPunit. TDD allows you to build-up the Behat skelton classes in the most simplistic, step by step manner. It focuses your thinking and helps you not get side-tracked with all the new things regarding OOP going through your head. It also ( done right ) will allow you to have flexible and decoupled designs.
You should also invest time in learning code patterns. Martin Fowler ( martinfowler.com ), is a great writer and mind when it comes to OOP and design processes. You would benefit from reading refactoring and code books and online material of Martin.
Another author you may want to explore is Fabian Potencier ( http://fabien.potencier.org/ ). Fabian is an active author in the PHP domain and is a skilled designer. He has authored the Symphony framework and smaller projects like Pimple ( Dependency Injection Container for PHP ). You may want to study the code of Symphony and other projects Fabian has created as it will teach you a lot regarding OOP.
There are people in this forum that are very good at OOP but to best leverage their knowledge it best serves you to invest in learning fundamentals and asking questions relative to concepts and technologies that you face along this journey.