Extending classes in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to extend a class in PHP?

Extending a class means creating a new class that inherits properties and methods from an existing class. This helps reuse code and add new features.

beginner

How do you declare a class that extends another class in PHP?

Use the <code>extends</code> keyword. For example: <br><code>class ChildClass extends ParentClass { }</code>

beginner

Can a child class override a method from its parent class?

Yes, a child class can provide its own version of a method defined in the parent class by defining a method with the same name.

intermediate

What keyword allows a child class to call the parent class's method it overrides?

The parent:: keyword is used to call the parent class's method from the child class.

beginner

Why is extending classes useful in programming?

It helps avoid repeating code, makes programs easier to maintain, and allows adding or changing features without rewriting everything.

Which keyword is used to create a class that inherits from another class in PHP?

A uses

B implements

C inherits

D extends

The extends keyword is used to inherit from a parent class in PHP.

If a child class has a method with the same name as its parent, what happens when you call that method on the child?

A The parent's method runs

B An error occurs

C The child's method runs

D Both methods run automatically

The child's method overrides the parent's method and runs instead.

How can a child class call the original method from its parent class it has overridden?

A Using <code>super::method()</code>

B Using <code>parent::method()</code>

C Using <code>this::method()</code>

D It cannot call the parent's method

The parent::method() syntax calls the parent class's method.

Which of these is NOT a benefit of extending classes?

A Slower program execution

B Easier maintenance

C Code reuse

D Adding new features easily

Extending classes usually does not slow down programs; it helps organize code better.

What happens if a child class does not define a method that exists in the parent class?

A The parent's method is used

B The method is ignored

C The program crashes

D You must define the method in the child

If the child class lacks the method, PHP uses the parent's method automatically.

Explain in your own words what it means to extend a class in PHP and why it is useful.

Think about how a child class can use and change what the parent class has.

Describe how method overriding works when extending classes in PHP and how to call the parent method from the child.

Focus on what happens when both classes have a method with the same name.

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