Dependency injection concept in PHP

Introduction

Dependency injection helps your code get what it needs without creating it inside. This makes your code easier to change and test.

When a class needs another class to work, like a car needs an engine.

When you want to change parts of your program without rewriting everything.

When you want to test parts of your program by giving fake parts instead of real ones.

When you want to keep your code clean and easy to understand.

Syntax

PHP

<?php
class Car {
    private $engine;

    public function __construct(Engine $engine) {
        $this->engine = $engine;
    }

    public function start() {
        $this->engine->run();
    }
}

class Engine {
    public function run() {
        echo "Engine is running";
    }
}

$engine = new Engine();
$car = new Car($engine);
$car->start();
?>

The Engine is given to Car from outside, not created inside Car.

This way, you can give Car different engines without changing Car code.

Examples

Logger is injected into User so User can log messages without creating Logger itself.

PHP

<?php
class Logger {
    public function log($message) {
        echo $message;
    }
}

class User {
    private $logger;

    public function __construct(Logger $logger) {
        $this->logger = $logger;
    }

    public function create() {
        $this->logger->log("User created\n");
    }
}

$logger = new Logger();
$user = new User($logger);
$user->create();

Shop depends on a payment method but does not create it. You can inject any payment gateway that follows the interface.

PHP

<?php
interface PaymentGateway {
    public function pay($amount);
}

class Paypal implements PaymentGateway {
    public function pay($amount) {
        echo "Paid $amount using Paypal\n";
    }
}

class Shop {
    private $paymentGateway;

    public function __construct(PaymentGateway $paymentGateway) {
        $this->paymentGateway = $paymentGateway;
    }

    public function checkout($amount) {
        $this->paymentGateway->pay($amount);
    }
}

$paypal = new Paypal();
$shop = new Shop($paypal);
$shop->checkout(100);

Sample Program

This example shows how the Car class gets an Engine from outside. The Car does not create the Engine itself. This is dependency injection.

PHP

<?php
// Engine class
class Engine {
    public function run() {
        echo "Engine is running\n";
    }
}

// Car class depends on Engine
class Car {
    private $engine;

    public function __construct(Engine $engine) {
        $this->engine = $engine;
    }

    public function start() {
        $this->engine->run();
    }
}

// Create Engine outside
$engine = new Engine();

// Inject Engine into Car
$car = new Car($engine);

// Start the car
$car->start();

Important Notes

Dependency injection makes your code easier to test because you can give fake parts.

It helps keep your code clean and flexible.

Summary

Dependency injection means giving a class what it needs from outside.

This helps change parts easily and test them separately.

It keeps your code clean and simple to understand.

Practice

1. What is the main idea behind dependency injection in PHP?

easy

A. Providing a class with its required objects from outside instead of creating them inside.

B. Writing all code inside one big class for simplicity.

C. Using global variables to share data between classes.

D. Making classes depend on static methods only.

Solution

1. Step 1: Understand dependency injection meaning

   Dependency injection means giving a class the objects it needs from outside, not creating them inside the class.

2. Step 2: Compare options with this meaning

   Only Providing a class with its required objects from outside instead of creating them inside. describes this idea correctly. Others describe bad or unrelated practices.

3. Final Answer:

   Providing a class with its required objects from outside instead of creating them inside. -> Option A

4. Quick Check:

   Dependency injection = Provide dependencies from outside [OK]

Hint: Dependency injection means passing needed parts from outside [OK]

Common Mistakes:

• Thinking dependency injection means using global variables
• Confusing dependency injection with static methods
• Believing all code should be inside one class

2. Which of the following is the correct syntax to inject a dependency via constructor in PHP?

easy

A. public function __construct(Database $db) { $this->db == $db; }

B. public function __construct() { $this->db = new Database(); }

C. public function __construct(Database $db) { $this->db = $db; }

D. public function __construct(Database $db) { $db = $this->db; }

Solution

1. Step 1: Identify correct constructor injection syntax

   Constructor injection requires passing the dependency as a parameter and assigning it to a class property.

2. Step 2: Check each option

   public function __construct(Database $db) { $this->db = $db; } correctly assigns the passed $db to $this->db. public function __construct() { $this->db = new Database(); } creates the dependency inside, not injection. public function __construct(Database $db) { $db = $this->db; } assigns backwards. public function __construct(Database $db) { $this->db == $db; } uses wrong operator '=='.

3. Final Answer:

   public function __construct(Database $db) { $this->db = $db; } -> Option C

4. Quick Check:

   Constructor injection = assign parameter to property [OK]

Hint: Assign constructor parameter to property with =, not == [OK]

Common Mistakes:

• Creating dependency inside constructor instead of injecting
• Using == instead of = for assignment
• Assigning property to parameter instead of reverse

3. Given the code below, what will be the output?

class Logger {
  public function log($msg) { echo "Log: $msg"; }
}

class User {
  private $logger;
  public function __construct(Logger $logger) {
    $this->logger = $logger;
  }
  public function create() {
    $this->logger->log('User created');
  }
}

$logger = new Logger();
$user = new User($logger);
$user->create();

medium

A. User created

B. Log: User created

C. Error: Logger not found

D. No output

Solution

1. Step 1: Understand dependency injection in code

   The User class receives a Logger object via constructor and stores it in $this->logger.

2. Step 2: Trace method calls

   Calling $user->create() calls $this->logger->log('User created'), which echoes "Log: User created".

3. Final Answer:

   Log: User created -> Option B

4. Quick Check:

   Injected Logger logs message correctly [OK]

Hint: Injected objects run their methods as expected [OK]

Common Mistakes:

• Expecting just 'User created' without 'Log:' prefix
• Thinking code throws error due to missing Logger
• Assuming no output without echo

4. Identify the error in this dependency injection example:

class Mailer {
  public function send($msg) { echo "Sending: $msg"; }
}

class Notification {
  private $mailer;
  public function __construct() {
    $this->mailer = $mailer;
  }
  public function notify() {
    $this->mailer->send('Hello');
  }
}

$notification = new Notification();
$notification->notify();

medium

A. The notify method should be static.

B. The send method is missing a return statement.

C. The Mailer class is not instantiated anywhere.

D. The constructor does not accept Mailer as a parameter to inject.

Solution

1. Step 1: Check constructor parameters

   The constructor has no parameters but tries to assign $mailer which is undefined inside it.

2. Step 2: Identify missing injection

   Mailer object should be passed as a parameter to constructor and assigned to $this->mailer.

3. Final Answer:

   The constructor does not accept Mailer as a parameter to inject. -> Option D

4. Quick Check:

   Constructor missing dependency parameter [OK]

Hint: Constructor must accept dependency parameter to inject it [OK]

Common Mistakes:

• Forgetting to pass dependency to constructor
• Trying to assign undefined variable inside constructor
• Thinking notify must be static

5. You want to make a ReportGenerator class that depends on a DataFetcher and a Logger. Which is the best way to apply dependency injection here?

hard

A. Pass both DataFetcher and Logger as constructor parameters and assign them to properties.

B. Create new DataFetcher and Logger objects inside ReportGenerator methods.

C. Use global variables for DataFetcher and Logger inside ReportGenerator.

D. Make ReportGenerator extend DataFetcher and Logger classes.

Solution

1. Step 1: Understand multiple dependency injection

   When a class needs multiple dependencies, best practice is to pass them all via constructor parameters.

2. Step 2: Evaluate options

   Pass both DataFetcher and Logger as constructor parameters and assign them to properties. correctly injects both dependencies. Create new DataFetcher and Logger objects inside ReportGenerator methods. creates dependencies inside, breaking injection. Use global variables for DataFetcher and Logger inside ReportGenerator. uses globals, which is bad practice. Make ReportGenerator extend DataFetcher and Logger classes. tries multiple inheritance which PHP does not support.

3. Final Answer:

   Pass both DataFetcher and Logger as constructor parameters and assign them to properties. -> Option A

4. Quick Check:

   Inject all dependencies via constructor [OK]

Hint: Inject all needed parts via constructor parameters [OK]

Common Mistakes:

• Creating dependencies inside methods instead of injecting
• Using global variables instead of injection
• Trying to extend multiple classes in PHP