You may start with one of the Object Oriented PHP tutorials or by practicing.
Practice | Tutorials
Classes and objects
Take the first steps by learning about classes, objects, methods, and properties.
The $this keyword
Learn how to use the class's own properties and methods from within the class with the $this keyword.
Chaining methods & properties
Learn how to chain your class's methods and properties to create much more streaming code.
Public vs. Private
Learn how to restrict the access to your classes code by using the private and protected access modifiers.
Magic methods and Constants
Learn how to write a constructor method without risking an error and how to use magic constants.
Inheritance in PHP
Learn how to reduce code duplication with inheritance, one of the most important concepts in object-oriented programming.
Abstract classes & methods
Learn how to commit child classes to certain methods with abstract classes and methods.
Interfaces - the next level of abstraction
Learn another way to commit the child classes to abstract methods that they should implement.
Learn about the Polymorphism naming convention that can help us write a much more coherent and easy to use code.