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Classes and Objects in PHP

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PHP extends keyword

In PHP, to define a class that inherits from another, we use the keyword extends:

class ChildClass extends ParentClass {
}

The newly defined class can access members with public and protected visibility from the base class, but cannot access private members.

The newly defined class can also redefine or override class members.

// Dog class inherits from Pet class. 
class Dog extends Pet {
  function bark() {
    return "woof";
  }
}

PHP Constructor Method

A constructor method is one of several magic methods provided by PHP. This method is automatically called when an object is instantiated. A constructor method is defined with the special method name __construct. The constructor can be used to initialize an object’s properties.

// constructor with no arguments:
class Person {
  public $favorite_color;
  function __construct() {
    $this->favorite_color = "blue";
  }
}

// constructor with arguments:
class Person {
  public $favorite_color;
  function __construct($color) {
    $this->favorite_color = $color;
  }
}

PHP class

In PHP, classes are defined using the class keyword.

Functions defined within a class become methods and variables within the class are considered properties.

There are three levels of visibility for class members:

  • public (default) - accessible from outside of the class
  • protected - only accessible within the class or its descendants
  • private - only accessible within the defining class

Members can be defined to be static:

  • Static members are accessed using the Scope Resolution Operator (::)

Classes are instantiated into objects using the new keyword. Members of an object are accessed using the Object Operator (->).

// Test class 
class Test {
  public $color = "blue";
  protected $shape = "sphere";
  private $quantity = 10;
  public static $number = 42;
  public static function returnHello() {
    return "Hello";
  }
}

// instantiate new object
$object = new Test();

// only color can be accessed from the instance
echo $object->color; # Works
echo $object->shape; # Fails
echo $object->quantity; # Fails
echo $object->number; # Fails

// we use the static class to access number
echo Test::$number;

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