Dependency injection concept in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Dependency Injection in simple terms?

Dependency Injection means giving an object what it needs instead of making it create those things itself. It helps keep code clean and easy to change.

beginner

Why is Dependency Injection useful?

It makes code easier to test, change, and reuse because parts don’t depend tightly on each other. You can swap parts without changing the whole system.

intermediate

In PHP, what is a common way to implement Dependency Injection?

You pass the needed objects as parameters to a class constructor or method instead of creating them inside the class.

intermediate

What is the difference between Dependency Injection and Service Locator?

Dependency Injection gives dependencies directly to a class, while Service Locator asks a central place to get dependencies. DI is clearer and easier to test.

beginner

Show a simple PHP example of Dependency Injection using constructor injection.

class Logger {
    public function log($msg) {
        echo $msg . "\n";
    }
}

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();

What does Dependency Injection help to achieve in code?

A Loose coupling between components

B Hard coding dependencies inside classes

C Making code run faster

D Avoiding use of objects

Dependency Injection helps make components less dependent on each other, which is called loose coupling.

Which of these is a common way to inject dependencies in PHP?

A Using global variables

B Passing dependencies via constructor

C Creating dependencies inside methods

D Using static methods only

Passing dependencies through the constructor is a common and clean way to do Dependency Injection in PHP.

What is NOT a benefit of Dependency Injection?

A Tighter coupling

B Easier testing

C Better code reuse

D Simpler maintenance

Dependency Injection reduces coupling, it does not make it tighter.

In Dependency Injection, who is responsible for creating the dependencies?

A The operating system

B The dependent class itself

C The PHP interpreter

D The dependency injection container or caller

Dependencies are created outside and passed in, often by a container or the code that uses the class.

Which pattern is considered clearer and easier to test?

A Service Locator

B Singleton

C Dependency Injection

D Factory

Dependency Injection is clearer and easier to test compared to Service Locator.

Explain Dependency Injection and why it is helpful in PHP programming.

Think about how objects get what they need without creating it themselves.

Describe how you would implement Dependency Injection in a PHP class with an example.

Focus on how dependencies are passed in and used inside the class.

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