Interface declaration and implementation in PHP - Mini Project: Build & Apply

Interface declaration and implementation

📖 Scenario: You are building a simple system to manage different types of vehicles. Each vehicle must be able to start its engine. You will use an interface to ensure all vehicles have a startEngine method.

🎯 Goal: Create an interface called Vehicle with a method startEngine. Then create two classes, Car and Motorcycle, that implement this interface. Each class should have its own version of the startEngine method. Finally, create objects of these classes and call their startEngine methods to see the output.

📋 What You'll Learn

Create an interface named Vehicle with a method startEngine.

Create a class Car that implements the Vehicle interface.

Create a class Motorcycle that implements the Vehicle interface.

Each class must have its own startEngine method that prints a unique message.

Create objects of Car and Motorcycle and call their startEngine methods.

💡 Why This Matters

🌍 Real World

Interfaces are used in real-world software to define common behaviors that different types of objects must have, like vehicles all needing to start their engines.

💼 Career

Understanding interfaces is important for writing clean, maintainable code and is a common requirement in many programming jobs, especially in object-oriented programming.

1

Create the Vehicle interface

Create an interface called Vehicle with a method declaration public function startEngine();.

PHP

<?php

// Create interface Vehicle with method startEngine
// Your code here

Hint

Use the interface keyword followed by the interface name. Inside, declare the method startEngine without a body.

2

Create the Car class implementing Vehicle

Create a class called Car that implements the Vehicle interface. Define the method startEngine inside Car to print "Car engine started".

PHP

<?php

interface Vehicle {
    public function startEngine();
}

// Create class Car implementing Vehicle
// Your code here

Hint

Use class Car implements Vehicle. Inside, define startEngine method that uses echo to print the message.

3

Create the Motorcycle class implementing Vehicle

Create a class called Motorcycle that implements the Vehicle interface. Define the method startEngine inside Motorcycle to print "Motorcycle engine started".

PHP

<?php

interface Vehicle {
    public function startEngine();
}

class Car implements Vehicle {
    public function startEngine() {
        echo "Car engine started";
    }
}

// Create class Motorcycle implementing Vehicle
// Your code here

Hint

Similar to Car, create Motorcycle class implementing Vehicle and define startEngine method with the correct message.

4

Create objects and call startEngine methods

Create an object $car of class Car and an object $motorcycle of class Motorcycle. Call startEngine on both objects, printing each result on a new line.

PHP

<?php

interface Vehicle {
    public function startEngine();
}

class Car implements Vehicle {
    public function startEngine() {
        echo "Car engine started";
    }
}

class Motorcycle implements Vehicle {
    public function startEngine() {
        echo "Motorcycle engine started";
    }
}

// Create objects and call startEngine methods
// Your code here

Hint

Create objects using new keyword. Call methods with ->. Use echo "\n"; to print a new line between outputs.

Practice

1. What is the main purpose of an interface in PHP?

easy

A. To execute code automatically when a program runs

B. To store variables and their values

C. To define method names without their code, ensuring classes implement them

D. To create objects directly

Solution

1. Step 1: Understand what an interface is

   An interface in PHP only declares method names without any code inside them.

2. Step 2: Understand the purpose of interfaces

   Interfaces ensure that any class implementing them must write the declared methods, helping organize code.

3. Final Answer:

   To define method names without their code, ensuring classes implement them -> Option C

4. Quick Check:

   Interface purpose = method declaration only [OK]

Hint: Interfaces declare methods only, no code inside [OK]

Common Mistakes:

• Thinking interfaces store variables
• Believing interfaces create objects
• Confusing interfaces with classes

2. Which of the following is the correct way to declare an interface named Vehicle in PHP?

easy

A. class Vehicle { public function start(); }

B. interface Vehicle { public function start(); }

C. interface Vehicle { function start() {} }

D. interface Vehicle { public function start() {} }

Solution

1. Step 1: Recall interface method declaration rules

   In interfaces, methods are declared without code blocks (no curly braces with code).

2. Step 2: Check each option

   'interface Vehicle { public function start(); }' declares interface and method without code, which is correct. Declarations with method bodies '{}' are wrong. Using 'class' instead of 'interface' is incorrect.

3. Final Answer:

   interface Vehicle { public function start(); } -> Option B

4. Quick Check:

   Interface methods have no body [OK]

Hint: Interface methods end with semicolon, no braces [OK]

Common Mistakes:

• Adding method bodies inside interfaces
• Using class keyword instead of interface
• Omitting semicolon after method declaration

3. What will be the output of this PHP code?

interface Animal {
  public function sound();
}

class Dog implements Animal {
  public function sound() {
    return "Bark";
  }
}

$dog = new Dog();
echo $dog->sound();

medium

A. Bark

B. sound

C. Animal

D. Fatal error

Solution

1. Step 1: Understand interface and class implementation

   The interface Animal declares method sound(). Dog class implements Animal and defines sound() returning "Bark".

2. Step 2: Check the output of calling $dog->sound()

   Calling $dog->sound() returns "Bark" which is then echoed.

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Implemented method returns "Bark" [OK]

Hint: Implemented methods run normally, output their return [OK]

Common Mistakes:

• Expecting interface name as output
• Confusing method name with output
• Thinking code causes error without implementation

4. Identify the error in this PHP code:

interface Shape {
  public function area();
}

class Circle implements Shape {
  public function area() {
    return 3.14 * 5 * 5;
  }

  public function perimeter() {
    return 2 * 3.14 * 5;
  }
}

$circle = new Circle();
echo $circle->area();

medium

A. No error, code runs and outputs area

B. Missing implementation of perimeter() in interface

C. Interface method area() has no body, causing error

D. Class Circle must implement all interface methods

Solution

1. Step 1: Check interface and class methods

   Interface Shape declares area(). Circle implements area() correctly and adds extra method perimeter() which is allowed.

2. Step 2: Verify if code runs without error

   Since all interface methods are implemented, extra methods are allowed. Code outputs area value.

3. Final Answer:

   No error, code runs and outputs area -> Option A

4. Quick Check:

   Extra methods allowed, interface methods implemented [OK]

Hint: Extra methods in class are allowed beyond interface [OK]

Common Mistakes:

• Thinking extra methods must be in interface
• Confusing method body rules in interface
• Assuming missing interface methods cause error here

5. You have two interfaces:

interface Printer {
  public function printDocument();
}

interface Scanner {
  public function scanDocument();
}

How can you create a class AllInOne that implements both interfaces and provides method bodies?

hard

A. class AllInOne implements Scanner {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

B. class AllInOne implements Printer {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

C. class AllInOne {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

D. class AllInOne implements Printer, Scanner {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

Solution

1. Step 1: Understand multiple interface implementation

   PHP allows a class to implement multiple interfaces by listing them separated by commas.

2. Step 2: Check which option correctly implements both interfaces

   class AllInOne implements Printer, Scanner { public function printDocument() { return "Printing..."; } public function scanDocument() { return "Scanning..."; } } implements both Printer and Scanner and defines both required methods. Others miss one interface or do not implement any.

3. Final Answer:

   class AllInOne implements Printer, Scanner { public function printDocument() { return "Printing..."; } public function scanDocument() { return "Scanning..."; } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with commas [OK]

Hint: Use commas to implement multiple interfaces in one class [OK]

Common Mistakes:

• Implementing only one interface when multiple required
• Forgetting to implement all interface methods
• Not using commas to separate interfaces