Classes

A class is a definition of the properties and methods of a particular type of object. When a class is used to define a new object, the resultant object is referred to as an instance of that class.

In a PHP class definition, properties are variables, methods are functions.

Syntax

<?php
class Car {
  // Properties
  public $make;
  public $model;
  public $year;

  // Methods
  function set_year($year) {
    $this->year = $year;
  }
  function get_year() {
    return $this->year;
  }
}
?>

This defines the class Car which has the properties make, model and year, and the methods set_year() and get_year().

The $this Keyword

Inside methods of a class, the $this keyword can be used to return a reference to the object instance that’s executing the method. An example of this is shown above. When the method refers to $this->year, it is referring to the year property of the particular object instance that is executing the method.

The new Keyword

An object is created from a class definition by using the new keyword.

Example

<?php
  class Car {
    // Properties
    public $make;
    public $model;
    public $year;

    // Methods
    function set_year($year) {
      $this->year = $year;
    }
    function get_year() {
      return $this->year;
    }
  }

  // Create object instances
  $honda = new Car();
  $ford = new Car();

  // Use class properties
  $honda->make = 'Honda';
  $honda->model = 'Accord';
  $ford->make = 'Ford';
  $ford->model = 'Explorer';

  // Use class methods
  $honda->set_year(2019);
  $ford->set_year(2020);

  echo $honda->get_year() . ' ' . $honda->make . ' ' . $honda->model . '<br/>';
  // Output: 2019 Honda Accord

  echo $ford->get_year() . ' ' . $ford->make . ' ' . $ford->model . '<br/>';
  // Output: 2020 Ford Explorer

?>

The instanceof Operator

The instanceof operator returns true when the operand on the left belongs to the class in the right operand.

Example

<?php
  $ford = new Car;
  if ($ford instanceof Car) {
    echo 'It is a Car!'; } else {
    echo 'It is not a Car!'; }
  // Output: It is a Car!
?>

Codebyte Example

The following code defines a Library class with attributes for library details, methods to display library information and register books with associated data. Try executing it:

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