Abstract classes and methods in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

What is the output of this PHP code?

PHP

<?php
abstract class Animal {
    abstract public function sound();

    public function describe() {
        return "This animal says: " . $this->sound();
    }
}

class Dog extends Animal {
    public function sound() {
        return "Woof";
    }
}

$dog = new Dog();
echo $dog->describe();
?>

A This animal says: sound

B Fatal error: Cannot instantiate abstract class Animal

C Woof

D This animal says: Woof

💡 Hint

Look at how the describe method uses the abstract sound method implemented in the subclass.

The abstract class Animal defines an abstract method sound and a concrete method describe that calls sound. The subclass Dog implements sound returning "Woof". So calling describe on a Dog instance returns "This animal says: Woof".

❓ Predict Output

intermediate

What error occurs when instantiating an abstract class?

What happens when you run this PHP code?

PHP

<?php
abstract class Vehicle {
    abstract public function move();
}

$car = new Vehicle();
?>

A No output, code runs fine

B Fatal error: Cannot instantiate abstract class Vehicle

C Parse error: syntax error, unexpected 'abstract'

D Warning: Abstract method move() not implemented

💡 Hint

Remember, abstract classes cannot be directly created as objects.

Abstract classes cannot be instantiated directly. Trying to create an object of an abstract class causes a fatal error.

🔧 Debug

advanced

Identify the error in abstract method implementation

What error will this PHP code produce?

PHP

<?php
abstract class Shape {
    abstract public function area();
}

class Circle extends Shape {
    public function area($radius) {
        return 3.14 * $radius * $radius;
    }
}

$circle = new Circle();
echo $circle->area(5);
?>

A Fatal error: Declaration of Circle::area($radius) must be compatible with Shape::area()

B Output: 78.5

C Parse error: syntax error, unexpected '$radius'

D Warning: Missing argument for area()

💡 Hint

Check if the method signatures match exactly between abstract and subclass methods.

The abstract method area in Shape has no parameters. The subclass Circle defines area with a parameter $radius. PHP requires the subclass method to have the same signature, so this causes a fatal error.

📝 Syntax

advanced

Which code snippet correctly defines an abstract class with an abstract method?

Choose the correct PHP code that defines an abstract class with one abstract method run.

A

<?php
abstract class Runner {
    abstract public function run();
}
?>

B

<?php
class Runner {
    abstract public function run();
}
?>

C

<?php
abstract class Runner {
    public abstract function run() {}
}
?>

D

<?php
abstract class Runner {
    abstract function run() {}
}
?>

💡 Hint

Remember abstract methods cannot have a body and must be inside an abstract class.

Option A correctly defines an abstract class Runner with an abstract method run without a body. Option A is invalid because abstract methods must be inside abstract classes. Options C and D incorrectly provide a method body for the abstract method, which is not allowed.

🚀 Application

expert

How many methods must be implemented?

Given this PHP code, how many methods must the class Smartphone implement to avoid errors?

PHP

<?php
abstract class Device {
    abstract public function powerOn();
    abstract public function powerOff();
    public function reset() {
        $this->powerOff();
        $this->powerOn();
    }
}

abstract class Phone extends Device {
    abstract public function call($number);
}

class Smartphone extends Phone {
    public function powerOn() {
        echo "Powering on";
    }

    public function powerOff() {
        echo "Powering off";
    }

    // Missing call method implementation
}
?>

A 3

B 2

C 1

D 0

💡 Hint

Count all abstract methods from parent classes that are not yet implemented.

The abstract class Device has two abstract methods: powerOn and powerOff, both implemented in Smartphone. The abstract class Phone adds one abstract method: call, which Smartphone has not implemented yet. So Smartphone must implement exactly one more method: call.

Practice

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

easy

A. To store data without any methods

B. To create objects directly without any child classes

C. To allow multiple inheritance in PHP

D. To provide a base class that cannot be instantiated directly

Solution

1. Step 1: Understand abstract class concept

   An abstract class is designed to be a base class that other classes extend but cannot be instantiated on its own.

2. Step 2: Eliminate incorrect options

   Options A, B, and D do not describe abstract classes correctly. PHP does not support multiple inheritance, and abstract classes cannot be instantiated directly.

3. Final Answer:

   To provide a base class that cannot be instantiated directly -> Option D

4. Quick Check:

   Abstract class = base class no direct objects [OK]

Hint: Abstract classes can't create objects directly [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Confusing abstract classes with interfaces
• Believing abstract classes store data only

2. Which of the following is the correct way to declare an abstract method in a PHP abstract class?

easy

A. abstract public function myMethod();

B. public function myMethod() {}

C. function abstract myMethod();

D. public abstract myMethod() {}

Solution

1. Step 1: Recall abstract method syntax

   In PHP, abstract methods are declared with the keyword abstract before the function signature and have no body (no curly braces).

2. Step 2: Check each option

   abstract public function myMethod(); correctly declares an abstract method: abstract public function myMethod();. Options B and D have bodies or wrong order, and C uses incorrect syntax.

3. Final Answer:

   abstract public function myMethod(); -> Option A

4. Quick Check:

   Abstract method = abstract + no body [OK]

Hint: Abstract methods have no body and use 'abstract' keyword [OK]

Common Mistakes:

• Adding curly braces {} to abstract methods
• Wrong keyword order or missing 'abstract'
• Trying to implement method body in abstract method

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

abstract class Animal {
    abstract public function sound();
}

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

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

medium

A. No output

B. Fatal error: Cannot instantiate abstract class Animal

C. Bark

D. Syntax error

Solution

1. Step 1: Understand class inheritance and method implementation

   The abstract class Animal has an abstract method sound(). The class Dog extends Animal and implements sound() returning "Bark".

2. Step 2: Check object creation and method call

   We create an object of Dog (which is allowed) and call sound(). This returns "Bark" which is echoed.

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Implemented abstract method returns 'Bark' [OK]

Hint: Only child classes can be instantiated, output from implemented method [OK]

Common Mistakes:

• Trying to instantiate abstract class directly
• Forgetting to implement abstract method in child
• Expecting syntax error due to abstract method

4. Identify the error in the following PHP code:

abstract class Vehicle {
    abstract public function startEngine();
}

class Car extends Vehicle {
    // No implementation of startEngine()
}

$car = new Car();
$car->startEngine();

medium

A. Class Car must implement abstract method startEngine()

B. Cannot instantiate abstract class Vehicle

C. Method startEngine() should not be abstract

D. No error, code runs fine

Solution

1. Step 1: Check abstract method implementation

   The abstract class Vehicle declares an abstract method startEngine(). The child class Car does not implement this method.

2. Step 2: Understand PHP rules for abstract methods

   PHP requires all abstract methods to be implemented in child classes unless the child is also abstract. Since Car is concrete and missing the method, this causes an error.

3. Final Answer:

   Class Car must implement abstract method startEngine() -> Option A

4. Quick Check:

   Missing abstract method implementation = error [OK]

Hint: All abstract methods must be implemented in child classes [OK]

Common Mistakes:

• Forgetting to implement abstract methods in child
• Trying to instantiate abstract class directly
• Assuming abstract methods can have bodies

5. Given the abstract class and two child classes below, what will be the output?

abstract class Shape {
    abstract public function area();
}

class Square extends Shape {
    private $side;
    public function __construct($side) {
        $this->side = $side;
    }
    public function area() {
        return $this->side * $this->side;
    }
}

class Circle extends Shape {
    private $radius;
    public function __construct($radius) {
        $this->radius = $radius;
    }
    public function area() {
        return 3.14 * $this->radius * $this->radius;
    }
}

$shapes = [new Square(4), new Circle(3)];
foreach ($shapes as $shape) {
    echo $shape->area() . " ";
}

hard

A. 62.82 61

B. 16 28.26

C. Error: Cannot instantiate abstract class Shape

D. 6 28.26

Solution

1. Step 1: Calculate area for Square

   Square side is 4, so area = 4 * 4 = 16.

2. Step 2: Calculate area for Circle

   Circle radius is 3, so area = 3.14 * 3 * 3 = 28.26.

3. Step 3: Understand output formatting

   The code echoes each area followed by a space, so output is "16 28.26 " without newline.

4. Final Answer:

   16 28.26 -> Option B

5. Quick Check:

   Area calculations correct, output matches [OK]

Hint: Calculate each area, watch for spaces and no newline [OK]

Common Mistakes:

• Confusing output formatting with newline
• Trying to instantiate abstract class Shape
• Incorrect area formulas