How to pass an array into a PHP SoapClient call

'stay' has to be defined just once. This should be the right answer:

$xml = array('reservation' => array(
'stays' => array(
    'stay' => array(
                    array(
                          'start_date' => '2011-01-01',
                          'end_date'   => 2011-01-15
                    ),
                    array(
                          'start_date' => '2011-01-01',
                          'end_date'   => 2011-01-15
                    )
              )  
    )
));

Assuming that when you instantiated $soapClient, you did so in WSDL mode, the following should work:

$stay1 = new stdClass();
$stay1->start_date = "2011-01-01";
$stay1->end_date = "2011-01-15";
$stay2 = new stdClass();
$stay2->start_date = "2011-01-01";
$stay2->end_date = "2011-01-15";
$stays = array();
$stays[0] = $stay1;
$stays[1] = $stay2;
$soapClient->saveReservation(
    array("reservation" => array("stays" => $stays))
);