Classes: Properties

A property is either a class-specific variable belonging to the class as a whole (in which case, it's declared static), or belonging to each instance of that class (in which case, static is absent). Each property has its own visibility. For example:

class Point {
  private static int $pointCount = 0; // static property with initializer
  private float $x; // instance property
  private float $y; // instance property

  public function __construct(num $x = 0, num $y = 0) { // instance method
    $this->x = (float)$x; // access instance property
    $this->y = (float)$y; // access instance property
    ++Point::$pointCount; // include new Point in Point count
  // ...

function main(): void {
  $p1 = new Point(3.4, 5.6);

Each instance of Point gets it own private set of x- and y-coordinates (the properties $x and $y, respectively), whose values are set by the constructor when new is used to create an instance. The initializer for an instance property, if any (there are none in this example), is applied prior to the constructor starting execution.

Being static, property $pointCount belongs to the class as a whole. It starts out at zero and is incremented by one by the constructor each time a new Point is created.

Note carefully, how we access the instance properties in the constructor, __construct. We cannot access them by using their short names, $x and $y. In any event, those would refer to local variables of that name, in this case, the method's parameters! When an instance method is called, the location of the Point on which it was called to operate is passed secretly to the called method inside of which it is available by the reserved name $this. So, to access property $x in the Point referred to by $this, we use the member-selection operator, ->, as in $this->x. There is no $ in front of the property name! (Static methods do not have a $this, so this approach is not used to access a static property, as we can see.)

Consider the case of a public property that we wish to access outside a method of its class. As we're not inside the class, no $this is available to us; however, we will have the name of the object containing that property, and we can use that along with the -> operator to select that property.

A static property exists regardless of whether any instances of its parent class exist.

By making the data implementation of a class private, we are practising data hiding, one of the basic principles of object-oriented programming.

All properties of non-nullable type must be initialized during object creation either by an explicit initializer or by the constructor, except that properties of nullable type that are not initialized either way take on the value null.

Constructor parameter promotion

Hack also supports a short-hand syntax for declaring a constructor that automatically assigns values of its parameters to the new instance's properties—see constructors.