<?php

/**
 * Like class is single, specify DIRECTIVE seems not be mandatory
 */
class Single extends ObjectivePHP\Config\SingleValueDirective {}

$config->import(new Single('x'))
       ->import(new Single('y'));
$config->get(Single::class) == "y";


/* ************************************************ */

class DbConf extends ObjectivePHP\Config\ObjectValueDirectiveGroup
{
    const host = 'string';
    const port = 'int';
    const username = 'string';
    const password = 'string';
    const dbname = 'string';

    public function toDsn()
    {
      return 'pgsql:host=' . $this->host
        . ';port=' . $this->port
        . ';dbname=' . $this->dbname
        . ';user=' .  $this->username
        . ';password=' . $this->password
      ;
    }
}

$config->import(new DbConf('dev', 'host', 'localhost'));
       ->import(new DbConf('dev', 'port', 5432));
       ->import(new DbConf('dev', 'dbname', 'myDb'));
       ->import(new DbConf('dev', 'user', 'foo'));
       ->import(new DbConf('dev', 'password', 'bar'));

$config->get(DbConf::class . '.dev', 'host') === 'localhost';

$dbConfDev = $config->subset(DbConf::class . '.dev')->toObject();
$dbConfDev->host === 'localhost';
$dbConfDev->port === 5432;
$dbConfDev->toDsn() === 'pgsql:host=localhost;port=5432;dbname=myDb;user=foo;password=bar';