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.