interfaces2.php

<?php

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Definition;
$container = new ContainerBuilder();
$factoryDefinition = new Definition('BarClassFactory');
$container
  ->setDefinition('barFactory', $factoryDefinition);
$definition = new Definition();
$definition
  ->setFactoryService('barFactory');
$definition
  ->setFactoryMethod('createBarClass');
$container
  ->setDefinition('bar', $definition);
return $container;
class BarClass {
  public $foo;
  public function setBar($foo) {
    $this->foo = $foo;
  }

}
class BarClassFactory {
  public function createBarClass() {
    return new BarClass();
  }

}