phpapprentice/code/static.php

<?php

// When writing a class, all of the properties and methods are being defined for the object
// that will be created from the class.

class House
{
    public $color;

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

// Like building a house, a class is a blueprint that
// defines what the house can do and the object is the house itself that can actually
// perform the actions defined in the blueprint.
$house = new House('Green');

// However, what if you want the blueprint to have properties and methods?
// That is when you use the "static" keyword. In this class, we will define a default color
// on the class itself and then use it when creating a new object.
class Skyscraper
{
    private static $popularColor;
    public $color;

    public static function setDefaultColor($color)
    {
        self::$popularColor = $color;
    }

    public function __construct()
    {
        $this->color = self::$popularColor;
    }
}

// You can access static methods and properties using double colons on "self" inside the object
// or on the class name outside of the object. Static methods and properties can only access
// other static methods and properties.
Skyscraper::setDefaultColor('Grey');
$skyscraper = new Skyscraper();
echo $skyscraper->color . "\n";

// Often, you will see static constructors in PHP.
// A static constructor creates a new instance of an object. Why would do that when you just use "new Class" to create
// a new object? The most common reason is to make the code more readable.
class TinyHouse
{
    private $color;
    private $wheels;
    private $trailer;

    public static function build($color, $wheels, $trailer)
    {
        return new self($color, $wheels, $trailer);
    }

    public function __construct($color, $wheels, $trailer)
    {
        $this->color = $color;
        $this->wheels = $wheels;
        $this->trailer = $trailer;
    }
}

// Using "build" can make more sense than "new", but it is ultimately a personal choice.
$house = TinyHouse::build('Blue', 4, true);