Why use such a construct in PHP?

Question

PHP

class Class_Name {
  private static $init;
  
  public static function instance() {
    if ( is_null( self::$init ) ) {
      self::$init = new self();
    }
    return self::$init;
  }

  private __construct() {

  }
}

Class_Name::instance();

That is, there is a check to see if there is already an instance of the class. Why this check if I create 1 instance using
new Class_Name();
In what cases can several objects be created or this check can not be done?

Answer 1

it's a classic singleton .
at what realized through .

Answer 2

It is used anywhere, if that