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

I have been reading about OOP recently and came across this private constructor scenario. I did a Google search, but couldn’t find anything relevant to PHP.

In PHP

• When do we have to define a private constructor?
• What’s the purpose of using a private constructor?
• What are the pros & cons of using a private constructor?

Answer 1

When do we have to define a private constructor?

class smt 
{
    private static $instance;
    private function __construct() {
    }
    public static function get_instance() {
        {
            if (! self::$instance)
                self::$instance = new smt();
            return self::$instance;
        }
    }
}

What’s the purpose of using a private constructor?

It ensures that there can be only one instance of a Class and provides a global access point to that instance and this is common with The Singleton Pattern.

What are the pros & cons of using a private constructor?

• Are Singletons really that bad?

• What is so bad about singletons?