In a PHP5 class, when does a private constructor get called?

Question

Let's say I'm writing a PHP (>= 5.0) class that's meant to be a singleton. All of the docs I've read say to make the class constructor private so the class can't be directly instantiated.

So if I have something like this:

class SillyDB
{
  private function __construct()
  {

  }

  public static function getConnection()
  {

  }
}

Are there any cases where __construct() is called other than if I'm doing a

new SillyDB() 

call inside the class itself?

And why am I allowed to instantiate SillyDB from inside itself at all?

Solution

__construct() would only be called if you called it from within a method for the class containing the private constructor. So for your Singleton, you might have a method like so:

class DBConnection
{
   private static $Connection = null;

   public static function getConnection()
   {
      if(!isset(self::$Connection))
      {
         self::$Connection = new DBConnection();
      }
      return self::$Connection;
   }

   private function __construct()
   {

   }
}

$dbConnection = DBConnection::getConnection();

The reason you are able/would want to instantiate the class from within itself is so that you can check to make sure that only one instance exists at any given time. This is the whole point of a Singleton, after all. Using a Singleton for a database connection ensures that your application is not making a ton of DB connections at a time.

---

Edit: Added $, as suggested by @emanuele-del-grande

Comments

[yodabar]

There's a little mistake when you refer to the static attribute Connection without the $ sign in self::Connection. This way, the PHP interpreter parses it as a constant, which, being not defined, would throw an error.