4.md

#Lesson 4: Static Properties

##New Keywords:

• static

##Introduction

Static methods exist only within the scope of the class, not the generated object. The only properties which they can access are properties which are also declared as static properties.

##Not Static Variables

You may have seen the use of static inside non-OO functions. Static variables keep their values across multiple function calls.

<?php

function test()
{
    static $a = 0;
    echo $a . "\n";
    $a++;
}

test();
test();

##Static Keyword in Classes

Declaring class properties or methods as static makes them accessible without needing an instantiation of the class.

An attribute declared as static cannot be accessed with an instantiated class object.

A method declared as static can be accessed with an instantiated class object.

<?php

class Dog {
  static $numLegs = 4;

  static function hasClaws() {
    return TRUE;
  }

  function bark() {
    echo "bow wow\n";
  }
}

class MyDog {
  static $numLegs = 3;

  function bark() {
    echo "bow ow\n";
  }
}

echo "Most dogs have " . Dog::$numLegs . " legs.\n";
echo "Can dogs scratch? " . Dog::hasClaws() . "\n";

$dog = new MyDog();

echo "My dog has " . MyDog::$numLegs . " legs.\n";
// Not
// echo "My dog has " . $dog->numLegs . " legs.\n";

###Exercise:

1. Add a static method to the "Person" class that compares two individuals by age and returns the oldest one.