Dependency injection concept in PHP - Mini Project: Build & Apply

Understanding Dependency Injection in PHP

📖 Scenario: You are building a simple PHP application that sends notifications. To keep your code clean and easy to change, you want to use dependency injection. This means you will give the notification sender to the notifier instead of making the notifier create it itself.

🎯 Goal: Learn how to use dependency injection by creating a notifier class that receives a sender object. You will create the sender, inject it into the notifier, and then send a notification message.

📋 What You'll Learn

Create a class called EmailSender with a method send that prints a message.

Create a class called Notifier that accepts a sender object in its constructor and stores it.

Add a method notify in Notifier that calls the sender's send method with a message.

Create an instance of EmailSender and inject it into Notifier.

Call the notify method to send a notification.

💡 Why This Matters

🌍 Real World

Dependency injection is used in many PHP frameworks to manage how objects get their dependencies, making code easier to test and maintain.

💼 Career

Understanding dependency injection is important for working with modern PHP frameworks like Laravel or Symfony, which use this pattern extensively.

1

Create the EmailSender class

Create a class called EmailSender with a public method send that takes a parameter $message and prints "Sending email: " followed by the message.

PHP

<?php

// Create the EmailSender class with a send method
// Your code here

Hint

Define a class with class EmailSender {}. Inside, add a public function send that uses echo to print the message.

2

Create the Notifier class with dependency injection

Create a class called Notifier with a private property $sender. Add a constructor that takes a parameter $sender and assigns it to the property $sender.

PHP

<?php

class EmailSender {
    public function send($message) {
        echo "Sending email: $message";
    }
}

// Create the Notifier class with a private property $sender and a constructor
// Your code here

Hint

Define a class Notifier with a private property $sender. The constructor should accept $sender and assign it to $this->sender.

3

Add notify method to call sender's send

In the Notifier class, add a public method notify that takes a parameter $message and calls $this->sender->send($message).

PHP

sender = $sender;
    }

    // Add notify method that calls sender's send
    // Your code here
}

Hint

Add a method notify that uses $this->sender->send($message) to send the message.

4

Create instances and send notification

Create an instance of EmailSender called $emailSender. Create an instance of Notifier called $notifier by injecting $emailSender. Call $notifier->notify("Hello World") to send the notification.

PHP

sender = $sender;
    }

    public function notify($message) {
        $this->sender->send($message);
    }
}

// Create EmailSender instance
// Create Notifier instance injecting EmailSender
// Call notify with "Hello World"
// Your code here

Hint

Create the sender and inject it into the notifier. Then call notify with the message.

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