Abstract classes and methods in PHP - Step-by-Step Execution

Concept Flow - Abstract classes and methods

Define abstract class

↓

Declare abstract method(s)

↓

Create subclass

↓

Implement abstract method(s)

↓

Instantiate subclass object

↓

Call implemented method(s)

↓

Program runs concrete behavior

This flow shows how an abstract class defines methods without code, subclasses must implement them, and then objects can be created to run concrete code.

Execution Sample

PHP

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

class Dog extends Animal {
  public function makeSound() {
    echo "Woof!\n";
  }
}

$dog = new Dog();
$dog->makeSound();
?>

Defines an abstract class Animal with an abstract method makeSound, then Dog implements it and outputs 'Woof!'.

Execution Table

Step	Action	Evaluation	Result
1	Define abstract class Animal with abstract method makeSound()	Class Animal created with abstract method	No object created yet
2	Define class Dog extends Animal	Dog must implement makeSound()	Class Dog created
3	Implement makeSound() in Dog	Method outputs 'Woof!'	Dog class ready for instantiation
4	Instantiate Dog object	$dog = new Dog()	Object $dog created
5	Call $dog->makeSound()	Executes Dog's makeSound()	Outputs: Woof!
6	End of program	No more code	Program ends

💡 Program ends after calling implemented abstract method in subclass

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
$dog	undefined	Object of class Dog	Object of class Dog	Object of class Dog

Key Moments - 3 Insights

Why can't we create an object of the abstract class Animal?

Because abstract classes are incomplete by design (see Step 1 in execution_table). They have abstract methods without code, so PHP forbids creating objects directly from them.

What happens if Dog does not implement the abstract method makeSound()?

PHP will give an error when defining Dog (Step 2) because all abstract methods must be implemented in subclasses.

Why does calling makeSound() on $dog output 'Woof!'?

Because Dog provides the concrete code for makeSound() (Step 5), so calling it runs Dog's version.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of $dog after Step 4?

A Object of class Dog

B Undefined

C Object of class Animal

D Null

💡 Hint

Check variable_tracker row for $dog after Step 4

At which step does the program output 'Woof!'?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at execution_table Action and Result columns for output

If Dog did not implement makeSound(), what would happen?

A Program runs normally

B Error when instantiating Dog

C Error when defining Dog class

D Output would be empty

💡 Hint

Refer to key_moments about missing implementation and Step 2 in execution_table

Concept Snapshot

abstract class ClassName {
  abstract public function methodName();
}

- Abstract classes cannot be instantiated.
- Abstract methods have no body and must be implemented in subclasses.
- Subclasses must define all abstract methods.
- Objects are created from subclasses, not abstract classes.

Full Transcript

This example shows how PHP uses abstract classes and methods. First, an abstract class Animal is defined with an abstract method makeSound. This method has no code and forces subclasses to implement it. Then, a subclass Dog extends Animal and provides the makeSound method that outputs 'Woof!'. We cannot create an Animal object directly because it is abstract. Instead, we create a Dog object and call its makeSound method, which prints 'Woof!'. This ensures a common interface while allowing different subclasses to provide their own behavior.

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