Login custom function for behat

login.php

/**
   * Authenticates a user with password from configuration.
   *
   * @Given /^I am logged in as user "([^"]*)"$/
   */
  public function iAmLoggedInAsUser($username) {
    $this->user = new stdClass();
    $this->user->name = $username;
    $this->user->pass = $this->drupal_users[$username];
    $this->login();
  }