PHP abstract Keyword

last modified April 16, 2025

The PHP abstract keyword is used to create abstract classes and methods in object-oriented programming. Abstract classes cannot be instantiated directly. They serve as base classes that other classes can extend.

Basic Definitions

An abstract class is a class that contains at least one abstract method. Abstract classes provide a common interface for their subclasses.

An abstract method is a method declared without implementation. It only defines the method signature. Concrete subclasses must implement all abstract methods.

The abstract keyword enforces a contract that derived classes must follow. This is a key concept in polymorphism and inheritance in PHP OOP.

Basic Abstract Class

This example demonstrates the simplest form of an abstract class in PHP.

basic_abstract.php

<?php

abstract class Animal {
    abstract public function makeSound();
}

class Dog extends Animal {
    public function makeSound() {
        echo "Bark";
    }
}

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

The Animal class is declared abstract with one abstract method. Dog extends Animal and implements makeSound. Attempting to instantiate Animal directly would cause an error.

Abstract Class with Concrete Methods

This example shows an abstract class with both abstract and concrete methods.

mixed_methods.php

<?php

abstract class Shape {
    abstract public function area();
    
    public function description() {
        return "This is a shape.";
    }
}

class Square extends Shape {
    private $side;
    
    public function __construct($side) {
        $this->side = $side;
    }
    
    public function area() {
        return $this->side * $this->side;
    }
}

$square = new Square(5);
echo $square->area();
echo $square->description();

Shape has an abstract area method and a concrete description method. Square implements area but inherits description. Abstract classes can mix method types.

Multiple Abstract Methods

This example demonstrates an abstract class with multiple abstract methods.

multiple_abstract.php

<?php

abstract class Database {
    abstract public function connect();
    abstract public function query($sql);
    abstract public function disconnect();
}

class MySQL extends Database {
    public function connect() {
        echo "Connecting to MySQL...";
    }
    
    public function query($sql) {
        echo "Executing: $sql";
    }
    
    public function disconnect() {
        echo "Disconnecting from MySQL...";
    }
}

$db = new MySQL();
$db->connect();
$db->query("SELECT * FROM users");
$db->disconnect();

The Database abstract class defines three abstract methods. MySQL must implement all of them. This ensures consistent interface across different database implementations.

Abstract Class with Properties

This example shows an abstract class with properties alongside abstract methods.

abstract_with_properties.php

<?php

abstract class Vehicle {
    protected $speed;
    
    abstract public function accelerate();
    
    public function getSpeed() {
        return $this->speed;
    }
}

class Car extends Vehicle {
    public function accelerate() {
        $this->speed += 10;
    }
}

$car = new Car();
$car->accelerate();
echo $car->getSpeed();

Vehicle has a protected property $speed and an abstract method. Car implements accelerate and can access the inherited property. Abstract classes can include properties and concrete methods.

Abstract Class Inheritance

This example demonstrates an abstract class extending another abstract class.

abstract_inheritance.php

<?php

abstract class A {
    abstract public function methodA();
}

abstract class B extends A {
    abstract public function methodB();
}

class C extends B {
    public function methodA() {
        echo "Method A implementation";
    }
    
    public function methodB() {
        echo "Method B implementation";
    }
}

$obj = new C();
$obj->methodA();
$obj->methodB();

B extends A and adds another abstract method. C must implement both abstract methods from both classes. Abstract classes can form inheritance hierarchies like concrete classes.

Abstract Class with Constructor

This example shows an abstract class with a constructor method.

abstract_constructor.php

<?php

abstract class Person {
    protected $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
    
    abstract public function greet();
}

class Student extends Person {
    public function greet() {
        return "Hello, I'm student {$this->name}";
    }
}

$student = new Student("Alice");
echo $student->greet();

Person has a concrete constructor that sets the $name property. Student calls the parent constructor and implements greet. Abstract classes can have constructors like regular classes.

Final Abstract Method

This example demonstrates combining abstract and final keywords in a method.

final_abstract.php

<?php

abstract class Template {
    final public function templateMethod() {
        $this->stepOne();
        $this->stepTwo();
    }
    
    abstract protected function stepOne();
    abstract protected function stepTwo();
}

class Implementation extends Template {
    protected function stepOne() {
        echo "Step 1 completed";
    }
    
    protected function stepTwo() {
        echo "Step 2 completed";
    }
}

$impl = new Implementation();
$impl->templateMethod();

templateMethod is final and defines an algorithm structure. The abstract methods stepOne and stepTwo must be implemented by subclasses. This is the Template Method design pattern.

Best Practices

Purpose: Use abstract classes to define common interfaces.
Implementation: All abstract methods must be implemented.
Design: Favor abstract classes when sharing code is needed.
Naming: Use clear names indicating abstract nature.
Documentation: Document expected behavior of abstract methods.

Source

PHP abstract Documentation

This tutorial covered PHP abstract classes and methods with practical examples showing abstract keyword usage in various scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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