You cannot serialize or unserialize PDO instances

if(!NS\Passwords::verify($mdp, $is_membre->password)) {
   $this->flashMessage('Vos identifiant et mot de passe ne coïncident pas.', 'failure');
} else {
   $this->flashMessage('YOUPI.', 'success');
   $user = $this->getUser();
   $user = iterator_to_array($user); // Greeny tip ?
   $user->login($is_membre->pseudo, $mdp);
   echo $user->isLoggedIn() ? 'yes' : 'no';
}

<?php

/**
 * This file is part of the Nette Framework (http://nette.org)
 * Copyright (c) 2004 David Grudl (http://davidgrudl.com)
 */

namespace Nette\Security;


/**
 * Performs authentication.
 */
interface IAuthenticator
{
	/** Credential key */
	const USERNAME = 0,
		PASSWORD = 1;

	/** Exception error code */
	const IDENTITY_NOT_FOUND = 1,
		INVALID_CREDENTIAL = 2,
		FAILURE = 3,
		NOT_APPROVED = 4;

	/**
	 * Performs an authentication against e.g. database.
	 * and returns IIdentity on success or throws AuthenticationException
	 * @return IIdentity
	 * @throws AuthenticationException
	 */
	function authenticate(array $credentials);

}

<?php

namespace App\Model;

use Nette;
use Nette\Security\Passwords;


/**
 * Users management.
 */
class UserManager extends Nette\Object implements Nette\Security\IAuthenticator
{
	const
		TABLE_NAME = 'membres',
		COLUMN_ID = 'id',
		COLUMN_NAME = 'pseudo', // changed my column since first message (nick->pseudo)
		COLUMN_PASSWORD_HASH = 'password',
		COLUMN_ROLE = 'role';


	/** @var Nette\Database\Context */
	private $database;


	public function __construct(Nette\Database\Context $database)
	{
		$this->database = $database;
	}


	/**
	 * Performs an authentication.
	 * @return Nette\Security\Identity
	 * @throws Nette\Security\AuthenticationException
	 */
	public function authenticate(array $credentials)
	{
		list($username, $password) = $credentials;

		$row = $this->database->table(self::TABLE_NAME)->where(self::COLUMN_NAME, $username)->fetch();

		if (!$row) {
			throw new Nette\Security\AuthenticationException('Le pseudo est incorrect.', self::IDENTITY_NOT_FOUND);

		} elseif (!Passwords::verify($password, $row[self::COLUMN_PASSWORD_HASH])) {
			throw new Nette\Security\AuthenticationException('Le mot de passe est incorrect.', self::INVALID_CREDENTIAL);

		} elseif (Passwords::needsRehash($row[self::COLUMN_PASSWORD_HASH])) {
			$row->update(array(
				self::COLUMN_PASSWORD_HASH => Passwords::hash($password),
			));
		}

		$arr = $row->toArray();
		unset($arr[self::COLUMN_PASSWORD_HASH]);
		return new Nette\Security\Identity($row[self::COLUMN_ID], $row[self::COLUMN_ROLE], $arr);
	}


	/**
	 * Adds new user.
	 * @param  string
	 * @param  string
	 * @return void
	 */
	public function add($username, $password)
	{
		try {
			$this->database->table(self::TABLE_NAME)->insert(array(
				self::COLUMN_NAME => $username,
				self::COLUMN_PASSWORD_HASH => Passwords::hash($password),
			));
		} catch (Nette\Database\UniqueConstraintViolationException $e) {
			throw new DuplicateNameException;
		}
	}

}



class DuplicateNameException extends \Exception
{}

namespace Nette\Security;

/**
 * Authentication exception.
 */
class AuthenticationException extends \Exception
{
}