Class config with Singleton parttern

Configuration.php

<?php
class Configuration {
    private static $instance;
    private function __construct($configuration) {/* */}
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new Configuration();
        }
        return self::$instance;
    }
    public function get($key) {/* */}
    public function getAll() {/* */}
}

$singleton = Configuration::getInstance();