Interface declaration and implementation in PHP - Step-by-Step Execution

Concept Flow - Interface declaration and implementation

Declare Interface

↓

Define Methods (no body)

↓

Create Class

↓

Implement Interface Methods

↓

Instantiate Class

↓

Call Implemented Methods

This flow shows how an interface is declared with method signatures, then a class implements those methods, and finally the class is used.

Execution Sample

PHP

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

class Dog implements Animal {
  public function makeSound() {
    echo "Woof!";
  }
}

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

This code declares an interface Animal with a method makeSound, then a Dog class implements it and outputs 'Woof!'.

Execution Table

Step	Action	Evaluation	Result
1	Declare interface Animal	Defines method makeSound()	Interface Animal ready
2	Define class Dog implementing Animal	Dog must implement makeSound()	Class Dog ready
3	Implement makeSound() in Dog	Method outputs 'Woof!'	Method implemented
4	Instantiate Dog object	$dog = new Dog()	Object $dog created
5	Call $dog->makeSound()	Executes method	Outputs: Woof!
6	End of script	No more code	Execution stops

💡 Script ends after method call outputs 'Woof!'

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 does the Dog class have to implement makeSound()?

Because the Dog class implements the Animal interface, it must provide the method makeSound() as declared in the interface (see execution_table step 2 and 3).

What happens if Dog does not implement makeSound()?

PHP will give an error because the class does not fulfill the interface contract (refer to execution_table step 2 where implementation is required).

Does the interface contain method bodies?

No, interfaces only declare method signatures without bodies (see execution_table step 1). The class provides the actual method code.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when $dog->makeSound() is called at step 5?

A Error

B Woof!

C Meow!

D No output

💡 Hint

Check execution_table row 5 for the output of makeSound()

At which step is the Dog object created?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at execution_table row 4 for object instantiation

If the Dog class did not implement makeSound(), what would happen at step 3?

A The script runs normally

B makeSound() outputs nothing

C PHP throws an error

D Dog class is abstract

💡 Hint

Refer to key_moments about missing method implementation

Concept Snapshot

interface InterfaceName {
  public function methodName();
}

class ClassName implements InterfaceName {
  public function methodName() {
    // method body
  }
}

- Interfaces declare method signatures only.
- Classes implementing interfaces must define all methods.
- Helps enforce consistent method names across classes.

Full Transcript

This example shows how to declare an interface in PHP with a method signature. The Dog class implements this interface and provides the method body. When we create a Dog object and call makeSound(), it outputs 'Woof!'. The interface ensures that any class implementing it must have the declared methods. If a class does not implement all interface methods, PHP will throw an error. Interfaces do not contain method bodies themselves, only declarations.