PHP Dependency Injection & Containers (Category 73)

PHPTrends Library Radar · Category 73

PHP Dependency Injection & Containers for Modern Teams

Dependency Injection (DI) is one of the quiet forces that decides whether your PHP codebase stays flexible or slowly turns into a ball of mud. Category 73 gives you a practical roadmap: understand DI, pick the right container, and roll it out without freezing development.

This page brings together a deep explanation of dependency injection in PHP, an overview of the most widely used containers, and a step-by-step adoption guide. It is written for technical leaders, architects and senior developers who want fewer hidden dependencies, fewer brittle tests and more predictable deployments.

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What dependency injection actually is

Dependency Injection is not a framework feature or a magic container. At its core it is a simple design rule: objects receive what they need instead of building it themselves. This makes object graphs explicit and moves wiring to the application edges where it belongs.

Without dependency injection

A tightly coupled controller might look like this. It decides how to construct the database connection, logger and service every time:

<?php

class ReportController
{
    public function generate(): string
    {
        $db     = new DatabaseConnection();
        $logger = new FileLogger('/var/log/app.log');

        $service = new ReportService($db, $logger);

        return $service->generateMonthlyReport();
    }
}

This looks harmless until you try to test or change it. Swapping the logger, moving to a different database driver or injecting a fake service for tests means touching the controller itself.

With constructor injection

With dependency injection, the controller describes what it needs and lets something else create those dependencies:

<?php

class ReportController
{
    public function __construct(
        private ReportService $service
    ) {}

    public function generate(): string
    {
        return $this->service->generateMonthlyReport();
    }
}

The construction logic is moved elsewhere: a factory, a framework’s service container or a standalone DI container. The controller becomes smaller, easier to reason about and considerably easier to test.

Dependency injection vs. dependency injection containers

It is important to separate the pattern from the tooling:

• Dependency Injection is the pattern of passing dependencies into objects.
• Dependency Injection Containers (often just “service containers”) are tools that automate object wiring and lifecycle management, especially in larger applications.

You can practice dependency injection with zero container. But once you have dozens of services with shared dependencies, a container becomes the difference between a clean bootstrap and an unmaintainable factory jungle.

Think of DI as the backbone of your application’s service hierarchy: explicit, discoverable and easy to move around.

Containers coordinate services, databases and infrastructure so your business logic can focus on behaviour instead of wiring.

PHP dependency injection containers at a glance

Modern PHP containers generally follow the PSR-11 Container interface and are built to be predictable, secure and interoperable. Below is a library radar for the containers you are most likely to encounter in real-world teams.

How to choose the right PHP DI container

You do not need to over-optimise this decision, but you should be deliberate. Use the following decision path as a quick guide:

A good rule of thumb: prioritise developer experience, documentation and community adoption over tiny performance differences. Containers rarely dominate runtime cost compared to I/O, databases and network calls.

Implementation blueprint: introducing DI without stopping development

Migrating an existing codebase to dependency injection does not have to be a risky “big bang” refactor. You can introduce DI incrementally while still shipping features.

1. Map your current services. List repositories, domain services, infrastructure adapters (HTTP clients, queues, caches) and note their current construction patterns.
2. Introduce constructor injection first. Before adding a container, refactor key classes so dependencies are passed in rather than created inside.
3. Create a composition root. Choose the entry points (front controllers, CLI commands, workers) where your container is set up and services are wired.
4. Avoid service locators. Let controllers and handlers receive their dependencies instead of pulling them from a global container.
5. Migrate global state. Wrap global functions, singletons and static access patterns behind interfaces, then inject those interfaces via DI.
6. Add tests as you go. Whenever you refactor a service to use DI, take the opportunity to add or improve tests around it.
7. Review lifecycle and performance. Use your container’s tools for singleton services, lazy loading or caching where it makes sense.

DI is not only a code-level pattern; it is a strategic choice about how your team reasons about dependencies and architecture.

Containers become the microchip at the centre of your application, routing dependencies consistently across commands, workers and HTTP requests.