php: When to use the scope resolution symbol (::) after an include / require_once?

All,

I'd like to understand

I'm looking at this code sample from w3Style's front controller tutorial:

index.php

<?php
define("PAGE_DIR", dirname(__FILE__) . "/pages");
require_once "FrontController.php";
FrontController::createInstance()->dispatch();

Why is the :: required in this case?

Does it have to do with the flexibility of one da开发者_JAVA百科y creating a different class in "FrontController.php" which would also have a method of this name? Or is it to address the case when one would have several includes of different classes, all incorporating some same method name?

Thanks,

JDelage

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FrontController seems to be a Singleton class.

The createInstance() method is called statically (hence the ::) and creates an object instance. Then, the dispatch() method is executed on the resulting object (hence the ->).

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It's because FrontController is a class with a createInstance() static method.

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When calling a class method which does not require a instance of the class (-> static call), you can call it by ::

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The guy tried to adopt the Singleton Pattern but there is mistake in his code:

This:

class FrontController {
  public static function createInstance() {
    if (!defined("PAGE_DIR")) {
      exit("Critical error: Cannot proceed without PAGE_DIR.");
    }
    $instance = new self();
    return $instance;
  }

Should become something like:

class FrontController {
  var $instance = NULL;
  public static function createInstance() {
    if (!defined("PAGE_DIR")) {
      exit("Critical error: Cannot proceed without PAGE_DIR.");
    }
    if($this->instance == NULL) {
      $this->instance = new self();
    }

    return $this->instance;
  }

That is just one approach but there are also different but similar. The idea is you have just one instance of this specific class and there is no duplication.