Extending classes in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extended class method call

What is the output of this PHP code?

PHP

<?php
class Animal {
    public function sound() {
        return "Some sound";
    }
}

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

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

A Some sound

B Fatal error: Cannot redeclare method

C Bark

D null

💡 Hint

Remember, when a child class defines a method with the same name as the parent, it overrides it.

The Dog class extends Animal and overrides the sound() method. So calling sound() on a Dog object returns "Bark".

❓ Predict Output

intermediate

Output when calling parent method from child

What will this PHP code output?

PHP

<?php
class Vehicle {
    public function start() {
        return "Vehicle started";
    }
}

class Car extends Vehicle {
    public function start() {
        return parent::start() . " and car is ready";
    }
}

$car = new Car();
echo $car->start();
?>

A Car started

B Vehicle started and car is ready

C Vehicle started

D Fatal error: Call to undefined method

💡 Hint

Look at how parent::start() is used inside the child method.

The Car class calls the parent start() method and appends additional text. So the output is "Vehicle started and car is ready".

❓ Predict Output

advanced

Output when child defines method same as parent's private

What is the output of this PHP code?

PHP

<?php
class Fruit {
    private function taste() {
        return "Sweet";
    }
}

class Apple extends Fruit {
    public function taste() {
        return "Sour";
    }
}

$apple = new Apple();
echo $apple->taste();
?>

A Sour

B Sweet

C Fatal error: Call to private method Fruit::taste() from context

D Fatal error: Cannot override private method

💡 Hint

Private methods are not inherited or overridable; child can define its own method with the same name.

The taste() method in Fruit is private and not inherited. Apple defines its own independent taste() method, which is called and outputs "Sour". No error occurs.

📝 Syntax

advanced

Which code causes a syntax error in class extension?

Which of the following PHP code snippets will cause a syntax error?

A <?php class A { public function foo() {} } class B extends A { public function foo() { parent.foo(); } } ?>

B <?php class A { public function foo() {} } class B extends A { public function foo() { echo 'Hi'; } } ?>

C <?php class A { public function foo() {} } class B extends A { public function foo() { parent::foo(); } } ?>

D <?php class A { public function foo() {} } class B extends A { public function foo() {} } ?>

💡 Hint

Check how the parent method is called inside the child method.

In PHP, to call a parent method, you must use parent::methodName(). Using parent.methodName() causes a syntax error.

🚀 Application

expert

Determine the number of methods available in child class

Given the following PHP classes, how many methods can be called on an instance of Smartphone?

PHP

<?php
class Device {
    public function powerOn() { return "Powering on"; }
    protected function reset() { return "Resetting"; }
    private function secret() { return "Secret"; }
}

class Phone extends Device {
    public function call() { return "Calling"; }
    protected function reset() { return "Phone reset"; }
}

class Smartphone extends Phone {
    public function browse() { return "Browsing"; }
    private function secret() { return "Smartphone secret"; }
}

$phone = new Smartphone();

A 4

B 6

C 5

D 3

💡 Hint

Count the public methods that can be called directly on a Smartphone instance from outside: powerOn(), call(), browse().

Public methods callable on a Smartphone instance from outside:
- powerOn() (from Device)
- call() (from Phone)
- browse() (from Smartphone)
Protected reset() cannot be called from outside. Private methods are inaccessible. Total: 3.

Practice

1. What keyword is used in PHP to create a child class that inherits from a parent class?

easy

A. extends

B. inherits

C. implements

D. uses

Solution

1. Step 1: Understand class inheritance in PHP

   PHP uses a specific keyword to let one class inherit properties and methods from another.

2. Step 2: Identify the correct keyword

   The keyword to extend a class in PHP is extends, not inherits, implements, or uses.

3. Final Answer:

   extends -> Option A

4. Quick Check:

   Keyword for inheritance = extends [OK]

Hint: Remember: PHP uses 'extends' to inherit classes [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Using 'uses' keyword incorrectly

2. Which of the following is the correct syntax to extend a class named Animal in PHP?

easy

A. class Dog extends Animal {}

B. class Dog inherits Animal {}

C. class Dog implements Animal {}

D. class Dog use Animal {}

Solution

1. Step 1: Recall PHP class extension syntax

   To create a child class, PHP uses the syntax: class Child extends Parent {}.

2. Step 2: Match the syntax with options

   class Dog extends Animal {} matches the correct syntax. The other options use incorrect keywords.

3. Final Answer:

   class Dog extends Animal {} -> Option A

4. Quick Check:

   Correct syntax uses 'extends' keyword [OK]

Hint: Use 'extends' keyword exactly after class name [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Using 'use' keyword for inheritance

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

class ParentClass {
    public function greet() {
        return "Hello from Parent";
    }
}

class ChildClass extends ParentClass {
    public function greet() {
        return "Hello from Child";
    }
}

$object = new ChildClass();
echo $object->greet();

medium

A. Hello from Parent

B. Fatal error

C. Hello from Child

D. No output

Solution

1. Step 1: Understand method overriding in child class

   The child class ChildClass overrides the greet() method of the parent class.

2. Step 2: Determine which method is called

   When calling greet() on an instance of ChildClass, the overridden method in the child class runs.

3. Final Answer:

   Hello from Child -> Option C

4. Quick Check:

   Child method overrides parent method [OK]

Hint: Child method overrides parent method with same name [OK]

Common Mistakes:

• Assuming parent method runs instead of child
• Expecting syntax error due to override
• Thinking no output will be printed

4. Identify the error in this PHP code that tries to extend a class:

class Vehicle {
    public function start() {
        return "Starting vehicle";
    }
}

class Car Vehicle {
    public function start() {
        return "Starting car";
    }
}

medium

A. Class Car should not have methods

B. Method start() cannot be overridden

C. Class Vehicle should be abstract

D. Missing 'extends' keyword before Vehicle

Solution

1. Step 1: Check class extension syntax

   In PHP, to extend a class, the child class must use the extends keyword.

2. Step 2: Identify the missing keyword

   The code class Car Vehicle {} misses extends between Car and Vehicle.

3. Final Answer:

   Missing 'extends' keyword before Vehicle -> Option D

4. Quick Check:

   Extending requires 'extends' keyword [OK]

Hint: Always write 'extends' when inheriting a class [OK]

Common Mistakes:

• Omitting 'extends' keyword
• Thinking methods cannot be overridden
• Confusing abstract class requirement

5. Given the following classes, what will be the output?

class Base {
    public function message() {
        return "Base message";
    }
}

class Derived extends Base {
    public function message() {
        return parent::message() . " and Derived message";
    }
}

class MoreDerived extends Derived {
    public function message() {
        return parent::message() . " plus MoreDerived message";
    }
}

$obj = new MoreDerived();
echo $obj->message();

hard

A. MoreDerived message

B. Base message and Derived message plus MoreDerived message

C. Base message

D. Fatal error due to parent call

Solution

1. Step 1: Trace method calls through inheritance chain

   MoreDerived::message() calls parent::message(), which is Derived::message().

2. Step 2: Understand Derived::message() call

   Derived::message() calls parent::message(), which is Base::message(), returning "Base message".

3. Step 3: Combine returned strings

   Base returns "Base message"; Derived appends " and Derived message"; MoreDerived appends " plus MoreDerived message".

4. Final Answer:

   Base message and Derived message plus MoreDerived message -> Option B

5. Quick Check:

   parent:: calls chain methods up inheritance [OK]

Hint: parent:: calls chain up inheritance to combine outputs [OK]

Common Mistakes:

• Ignoring parent:: calls and output only last method
• Expecting fatal error from parent calls
• Thinking only base method runs