$city = array(0 => 'Москва', 1 => 'Питер', 2 => 'Вологда');
list($city[0], $city[1]) = array($city[1], $city[0]);
print_r($city);

Array
(
    [0] => Питер
    [1] => Москва
    [2] => Вологда
)