A Comprehensive Guide to the “class” Keyword in PHP

The “class” keyword is a fundamental part of PHP, which allows you to create classes that can be used to create objects. These objects have properties and methods that define their characteristics and behaviour. You can also use the “class” keyword to create inheritance hierarchies, define abstract and final classes and methods, and implement interfaces. Understanding the “class” keyword is important for creating organised and reusable code in PHP.

Introduction to classes and objects in PHP

In object-oriented programming, a class is a blueprint for creating objects. An object is an instance of a class, and contains its own set of properties and methods.

For example:

class Car {
  // class properties and methods go here
}

$car1 = new Car();
$car2 = new Car();

In this example, a class called “Car” is defined. The “Car” class can contain properties and methods that define the characteristics and behaviour of a car. Two objects, $car1 and $car2, are created from the “Car” class using the “new” operator.

Defining a class in PHP

To define a class in PHP, you use the “class” keyword followed by the name of the class and a set of curly braces that enclose the class body.

For example:

class Car {
  // class properties and methods go here
}

In this example, a class called “Car” is defined. The class body is left empty, as the class does not contain any properties or methods yet.

Defining properties and methods in a class

To define properties in a class, you use the visibility modifier (such as “public” or “private”) followed by the “$” symbol and the name of the property.

For example:

class Car {
  public $make;
  public $model;
  public $year;
}

In this example, three public properties, $make, $model, and $year, are defined in the “Car” class. These properties can be accessed and modified from outside the class using object notation.

To define methods in a class, you use the visibility modifier (such as “public” or “private”) followed by the function keyword, the name of the method, and a set of parentheses.

For example:

class Car {
  // class properties go here

  public function startEngine() {
    // method code goes here
  }

To define methods in a class, you use the visibility modifier (such as “public” or “private”) followed by the function keyword, the name of the method, and a set of parentheses.

For example:

class Car {
  // class properties go here

  public function startEngine() {
    // method code goes here
  }
}

In this example, a public method called “startEngine” is defined in the “Car” class. This method can be called from outside the class using object notation.

Creating objects from a class

To create an object from a class, you use the “new” operator followed by the name of the class.

For example:

$car = new Car();

In this example, an object called $car is created from the “Car” class. The object contains its own set of properties and methods defined in the class.

Accessing and modifying object properties and methods

To access an object’s properties and methods, you use object notation, which consists of the object variable followed by the “->” operator and the property or method name.

For example:

$car->make = "Toyota";
$car->model = "Corolla";
$car->year = 2020;

echo $car->make . " " . $car->model . " (" . $car->year . ")"; // outputs "Toyota Corolla (2020)"

In this example, the properties of the $car object are accessed and modified using object notation. The “make”, “model”, and “year” properties are set to “Toyota”, “Corolla”, and 2020, respectively. The properties are then accessed and concatenated using string manipulation to output a string.

To call an object’s methods, you use object notation followed by a set of parentheses.

For example:

$car->startEngine();

In this example, the “startEngine” method of the $car object is called using object notation.

Class inheritance

PHP allows classes to inherit properties and methods from parent classes using the “extends” keyword. This allows you to create a base class that defines common characteristics and behavior, and create derived classes that inherit and override those characteristics and behavior as needed.

For example:

class Vehicle {
  public $numWheels;

  public function __construct($numWheels) {
    $this->numWheels = $numWheels;
  }

  public function getNumWheels() {
    return $this->numWheels;
  }
}

class Car extends Vehicle {
  public $make;
  public $model;
  public $year;

  public function __construct($make, $model, $year) {
    parent::__construct(4);
    $this->make = $make;
    $this->model = $model;
    $this->year = $year;
  }
}

$car = new Car("Toyota", "Corolla", 2020);
echo $car->getNumWheels(); // outputs "4"

In this example, a class called “Car” is defined that extends the “Vehicle” class and adds its own properties called “make”, “model”, and “year”. The “Car” class also defines its own constructor method that calls the parent class’s constructor method using the “parent::” keyword and sets its own properties.

Abstract classes and methods

PHP allows you to define abstract classes and methods, which are special types of classes and methods that do not have any implementation and must be implemented by concrete classes that extend or implement them.

An abstract class is defined using the “abstract” keyword followed by the “class” keyword and the name of the class. An abstract method is defined using the “abstract” keyword followed by the visibility modifier (such as “public” or “private”), the “function” keyword, the name of the method, and a set of parentheses.

For example:

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

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

$dog = new Dog();
$dog->speak(); // outputs "Bark!"

In this example, an abstract class called “Animal” is defined with an abstract method called “speak”. A concrete class called “Dog” is defined that extends the “Animal” class and implements the “speak” method. An object of the “Dog” class is created and the “speak” method is called, which outputs “Bark!”.

Interfaces

PHP also allows you to define interfaces, which are special types of classes that contain only abstract methods and constants, and cannot contain any implementation. An interface is defined using the “interface” keyword followed by the name of the interface and a set of curly braces that enclose the interface body.

An interface is implemented in a class using the “implements” keyword followed by the name of the interface. The class must then implement all of the abstract methods declared in the interface.

For example:

interface Animal {
  public function speak();
}

class Dog implements Animal {
  public function speak() {
    echo "Bark!";
  }
}

$dog = new Dog();
$dog->speak(); // outputs "Bark!"

In this example, an interface called “Animal” is defined with an abstract method called “speak”. A class called “Dog” is defined that implements the “Animal” interface and implements the “speak” method. An object of the “Dog” class is created and the “speak” method is called, which outputs “Bark!”.

In conclusion, the “class” keyword in PHP is used to define classes, which are templates for creating objects. Classes can contain properties and methods that define the characteristics and behavior of the objects created from them. PHP allows you to create inheritance hierarchies using the “extends” keyword, define abstract classes and methods using the “abstract” keyword, and implement interfaces using the “implements” keyword. By using these features of PHP, you can create more flexible and modular code.

Final classes and methods

PHP also allows you to define final classes and methods, which are classes and methods that cannot be extended or overridden by derived classes or objects.

A final class is defined using the “final” keyword followed by the “class” keyword and the name of the class. A final method is defined using the “final” keyword followed by the visibility modifier (such as “public” or “private”), the “function” keyword, the name of the method, and a set of parentheses.

For example:

final class Animal {
  final public function speak() {
    echo "I am an animal!";
  }
}

class Dog extends Animal {
  // cannot override speak() method
}

$dog = new Dog();
$dog->speak(); // outputs "I am an animal!"

In this example, a final class called “Animal” is defined with a final method called “speak”. A class called “Dog” is defined that extends the “Animal” class, but cannot override the “speak” method because it is marked as final. An object of the “Dog” class is created and the “speak” method is called, which outputs “I am an animal!”.

Final classes and methods can be useful for ensuring that certain behaviour or functionality is not changed or overridden by derived classes or objects. However, it is generally recommended to use abstract classes and methods whenever possible, as they allow for more flexibility and extensibility in your code.

I hope this tutorial helps you understand the use of the “class” keyword and related concepts in PHP. If you have any questions or need further clarification, please don’t hesitate to ask.