<?php

final class Login
{
    private $sessions;
    
    public function __construct(SessionRepository $sessions)
    {
        $this->sessions = $sessions;
    }
    
    public function __invoke(User $user)
    {
        $session = new Session($user);
        
        $this->sessions->add($session);
        
        return $session;
    }
}

// usage (PHP 7) - see http://3v4l.org/kPnLM:

$session = (new Login($sessionRepository))($user);