Make the clone/construct methods private and final, so I cannot do this:

false_singleton.php

<?php

abstract class Singleton {

  private function __construct() {
    // empty
  }

  public static function getInstance() {
    static $instance = null;
    return $instance ?: $instance = new static;
  }

  //attempt to prevent cloning of an object
  public function __clone() {
    trigger_error('Clone is not allowed.', E_USER_ERROR);
  }

}

//no longer a singleton... even though it extends singleton
class oops extends Singleton{
  public $test = 'test';
  public function __clone(){
    return $this;
  }
}

//no longer a singleton, see?
$oops = oops::getInstance();
$oopsClone = clone $oops;
$oopsClone->test = 'not test';
var_dump($oopsClone, $oops); // not the same oops

gistfile2.txt