Behat step for logging in

gistfile1.php

$steps->Given('/^I am logged into the admin area as rich_81$/', function($world) {

    $session = $world->getSession();
    $doc = $session->getPage();

    // Ensure we're logged out for cases where the session isn't
    // reset between Behat steps    
    $session->visit($world->locatePath("/admin/logout"));
    $session->visit($world->locatePath("/admin"));

    $doc->fillField("_username", "rich_81");
    $doc->fillField("_password", "foobar");
    $doc->clickButton("Log in");
});