Dependency injection concept in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject the Logger dependency into the UserService constructor.

PHP

<?php
class UserService {
    private $logger;
    public function __construct([1] $logger) {
        $this->logger = $logger;
    }
}

Drag options to blanks, or click blank then click option'

ALogger

BUserService

CDatabase

DConfig

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong class name in the constructor parameter.

Not specifying a type for the constructor parameter.

The constructor requires an instance of the Logger class to be injected. This is dependency injection.

2fill in blank

medium

Complete the code to create a new UserService instance with a Logger injected.

PHP

<?php
$logger = new [1]();
$userService = new UserService($logger);

Drag options to blanks, or click blank then click option'

AUserService

BLogger

CDatabase

DConfig

Attempts:

3 left

💡 Hint

Common Mistakes

Creating the wrong class instance.

Not creating an instance before injecting.

You must create a Logger object to inject it into UserService.

3fill in blank

hard

Fix the error in the constructor to properly assign the injected dependency.

PHP

<?php
class UserService {
    private $logger;
    public function __construct(Logger $logger) {
        $this->[1] = $logger;
    }
}

Drag options to blanks, or click blank then click option'

Alog

BlogService

Clogger

DLogger

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different property name than declared.

Using uppercase or incorrect casing.

The property name is $logger, so assign to $this->logger.

4fill in blank

hard

Fill both blanks to complete the setter method for dependency injection.

PHP

<?php
class UserService {
    private $logger;
    public function [1](Logger $logger) {
        $this->[2] = $logger;
    }
}

Drag options to blanks, or click blank then click option'

AsetLogger

BgetLogger

Clogger

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' instead of 'set' for the method name.

Assigning to a wrong property name.

The setter method is named setLogger and assigns to the property $logger.

5fill in blank

hard

Fill all three blanks to complete the code that uses dependency injection with interface and implementation.

PHP

<?php
interface LoggerInterface {
    public function log(string $message);
}

class FileLogger implements [1] {
    public function log(string $message) {
        // write to file
    }
}

class UserService {
    private $logger;
    public function __construct([2] $logger) {
        $this->logger = $logger;
    }
    public function doAction() {
        $this->logger->[3]("Action done");
    }
}

Drag options to blanks, or click blank then click option'

ALoggerInterface

Clog

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using the implementation class name instead of interface in constructor.

Calling a method name that does not exist.

The class FileLogger implements LoggerInterface. The constructor type hints LoggerInterface. The method called is log.

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