services.yml

services:  
    password_encoder:
        class: Symfony\Component\Security\Core\Tests\Encoder\PasswordEncoder
        factory_service: security.encoder_factory
        factory_method: getEncoder
        arguments:
            - Example\Model\User

UserManager.php

<?php  
namespace Example\Manager;

use JMS\DiExtraBundle\Annotation\Inject;  
use JMS\DiExtraBundle\Annotation\InjectParams;  
use JMS\DiExtraBundle\Annotation\Service;  
use Example\Model\User;  
use Symfony\Component\Security\Core\Encoder\PasswordEncoderInterface;

/**
 * @Service("user_manager")
 */
class UserManager  
{
    /**
     * @var PasswordEncoderInterface
     */
    private $passwordEncoder;

    /**
     * @InjectParams({
     *     "passwordEncoder" = @Inject("password_encoder")
     * })
     *
     * @param PasswordEncoderInterface $passwordEncoder
     */
    public function __construct(PasswordEncoderInterface $passwordEncoder)
    {
        $this->passwordEncoder = $passwordEncoder;
    }

    /**
     * @param User $user
     */
    public function save(User $user)
    {
        $password = $this->passwordEncoder->encodePassword($user->getPlainPassword(), $user->getSalt());
        $user->setPassword($password);
    }
}

UserManagerTest.php

<?php  
namespace Example\Manager;

use PHPUnit_Framework_TestCase;  
use Example\Model\User;  
use Symfony\Component\Security\Core\Encoder\PasswordEncoderInterface;

class UserManagerTest extends PHPUnit_Framework_TestCase  
{
    const PLAIN_PASSWORD = 'password';
    const ENCODED_PASSWORD = 'encoded-password';

    /**
     * @test
     */
    public function shouldEncodePasswordIfPlainPasswordExists()
    {
        $passwordEncoder = $this->getMock(PasswordEncoderInterface::class);
        $userManager = new UserManager($passwordEncoder);

        $user = new User([
            'plainPassword' => self::PLAIN_PASSWORD,
        ]);

        $passwordEncoder
            ->expects($this->once())
            ->method('encodePassword')
            ->with(self::PLAIN_PASSWORD, $user->getSalt())
            ->willReturn(self::ENCODED_PASSWORD);

        $userManager->save($user);
        $this->assertEquals(self::ENCODED_PASSWORD, $user->getPassword());
    }
}