Common Singleton pattern in php

Singleton.php

<?php

abstract class Singleton {
    
    protected function __construct() {}

    final public static function instance () {
        static $instances = array();
        $calledClass = get_called_class();
        if (!isset($instances[$calledClass])) {
            $instances[$calledClass] = new $calledClass();
        }
        return $instances[$calledClass];
    }

    final private function __clone() {}
}