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Classes vs. Objects

• A class provides a definition for a user-defined data type. An object is an instance of a class providing values to a class’s properties.
• Each instance of a class has their own ‘version’ of a class’s properties, and these are referred to as instance properties.
• Static properties are class properties whose values are shared between all of the instances of a particular class.
• A property is declared static in a class definition by using the static keyword:

  public static $numPeople = 0;

• An object of a class does NOT need to be instantiated to access a class's static properties.
• The scope resolution operator (::) is used to access static properties.
• A public static property is accessed from outside of a class definition using the syntax:

  Person::$numPeople; // uses the class name

• A static property, whether public or private, can be accessed inside a class definition with an alternative syntax that uses the self keyword:

  self::$numPeople;

• A static property CANNOT be accessed via an object instance variable using the -> operator.
• Static methods are methods defined in a class that are at the class-level and are NOT associated with particular instances of a class. They are declared with the static keyword:

  public static function add($op1, $op2) {…

• Static methods CANNOT use the $this variable within their definitions.
• Non-static methods are referred to as instance methods.
• Static methods are called using the scope resolution operator (externally using the class name, and internally using the self keyword):
  • Externally: Person::getClassName()
  • Internally: self::getClassName()
• A class constant is a constant defined within a class definition that is specific to that class. They are defined using the const keyword:

  const PI = 3.14;

• They are accessed using the scope resolution operator (with no $ prepending their name):
  • Externally: Math::PI
  • Internally: self::PI
• Additional Resources:
  • PHP Manual - static Properties & Methods
  • PHP Manual - Scope Resolution Operator (::)
  • PHP Manual - Class Constants