Dependency Injection in PHP

Introduction

The book "PHP Dependency Injection" (author — Sam Yong, 2019, 176 pages) is a compact yet comprehensive guide to one of the key principles of modern PHP development. Dependency Injection (DI) is the foundation of frameworks like Symfony, Laravel, Yii2, and many others. Without understanding DI, it is impossible to write testable, flexible, and maintainable code. Yong's book doesn't just explain the theory — it guides the reader from simple examples to creating their own DI container and integrating with popular libraries.

What the book is about

The publication consists of 10 chapters, each dedicated to a specific aspect of DI. The author starts with basic concepts: what a dependency is, why tight coupling is harmful, and how Inversion of Control (IoC) solves this problem. Then, the following topics are covered sequentially:

• Manual dependency injection via constructor, setters, and interfaces;
• Dependency injection containers: their structure, service registration, dependency resolution;
• The PSR-11 standard (Container Interface) and its implementation;
• Advanced techniques: autowiring, factories, decorators, service tagging;
• Integration of DI with popular PHP frameworks (Symfony, Laravel, Zend Framework);
• Testing code with DI: mocks, stubs, containers for tests;
• Refactoring legacy code using DI.

Each chapter contains practical code examples that can be run immediately. The author uses real-world scenarios: working with a database, logging, sending emails.

Who this book is for

The book is aimed at mid-level PHP developers who already have experience building web applications but want to improve code quality. Basic knowledge of OOP and PHP syntax is required. Beginners just learning PHP will find it difficult — the material assumes an understanding of classes, interfaces, exceptions, and autoloading. However, for those who already write in Laravel or Symfony "by the documentation" but don't fully understand the container's mechanisms, the book will be an excellent bridge to deep understanding.

What the reader will learn

• A clear distinction between Dependency Injection and Service Locator;
• How to design classes so they depend on abstractions, not concrete implementations;
• How to write your own simple DI container from scratch (only ~100 lines of code);
• How to use ready-made containers: PHP-DI, Symfony DependencyInjection, Pimple;
• How to automate dependency resolution using reflection and autowiring;
• How to organize service configuration (YAML, XML, PHP arrays);
• How to test classes with injected dependencies without creating heavy integration tests;
• How to gradually introduce DI into an existing project without a complete rewrite.

Strengths

1. Practical focus. There is no "fluff" in the book — each section is backed by code. The author doesn't just talk, but shows what bad code looks like and how to improve it step by step.
2. Gradual complexity. From manual injection to containers — the reader is not overloaded with complex concepts all at once.
3. Relevance. The book was published in 2019, but the principles of DI do not become outdated. All examples are compatible with PHP 7.x and 8.x (with minor adaptations).
4. Volume. 176 pages is an ideal format for quickly mastering the topic. The book can be read over a weekend and then used as a reference.
5. Focus on PSR-11. The author pays attention to the standard, making the knowledge transferable between frameworks.

Who it is recommended for

The book will be useful for:

• Developers who want to move from procedural or "spaghetti code" to a clean architecture;
• Those who use frameworks but want to understand their internal workings;
• Teams planning to refactor legacy projects;