This book discusses object-oriented programming methodologies in PHP and the application of key software design principles based on classic design patterns, as well as describing the tools and practices for development, testing, continuous integration, and deployment of reliable application code. This sixth edition of the book has been completely updated in accordance with version 8 of the PHP language. This book details the new features of PHP, such as attributes and numerous improvements in type declarations. The main goal of the book is to explore some established design principles and core design patterns in the context of PHP. Primarily, the book is aimed at developers who have a solid grasp of the fundamentals of programming in PHP and are looking to enhance their web application design skills by applying best development practices. This sixth edition of this popular book has been fully updated in accordance with version 8 of the PHP language and includes a description of the Composer dependency manager, material dedicated to the tools Vagrant, PHPUnit, Jenkins, and others, as well as recommendations for PHP programming standards. This book details the new features of PHP, such as attributes and numerous improvements in type declarations. The main goal of the book is to explore some established design principles and core design patterns in the context of PHP. Part I of this book covers the key object-oriented features of the PHP language, including class and type declarations, inheritance, reflection, and much more. Part II is dedicated to design patterns, explaining the principles that determine their effectiveness, as well as classic patterns for designing enterprise applications and databases. Part III discusses tools and practices that help turn quality code into a successful project. This part shows how to organize collaborative work among developers and software product releases using Git and how to manage the project build process and dependencies using Composer, as well as exploring strategies for automated testing and continuous integration. Overall, the book is dedicated to the fundamentals of object-oriented programming, design principles, and best development practices aimed at helping the reader develop elegant, reliable, and resilient systems.