$sql="SELECT Building, Room FROM Room WHERE Building = '".$building."'";

    $sqlresult = mysql_query($sql);

    $buildings = array(); // easier if you don't use generic names for data

    while($sqlrow = mysql_fetch_array($sqlresult))
    {
        // you need to initialise your building array cells
        if (!isset($buildings[$sqlrow['Building']])) {
            $buildings[$sqlrow['Building']] = array('Rooms' => array());
        }

        // you can add the room to the building 'Rooms' array
        $buildings[$sqlrow['Building']]['Rooms'][] = $sqlrow['Room'];
    }


    $buildingHTML = "";
    $buildingHTML = "<form action=\"\" method=\"post\">";
    $buildingHTML .= '<select name="buildings" id="buildingssDrop" onchange="document.getElementById(\'dropDownForm\').submit()">'.PHP_EOL;
    $buildingHTML .= '<option value="">Please Select</option>'.PHP_EOL;

    foreach ($buildings as $building => $buildingData) {
        $buildingHTML .= "<option value='".$building."'>" . $building . "</option>".PHP_EOL;
    }
    $buildingHTML .= '</select>';
    $buildingHTML .= '</form>';

    $roomHTML = "";
    $roomHTML .= '<select name="rooms" id="roomsDrop">'.PHP_EOL;
    $roomHTML .= '<option value="">Please Select</option>'.PHP_EOL;

    $buildingname = $_POST['buildings'];
    foreach ($buildings[$buildingname]['Rooms'] as $roomId => $roomData) {
        $roomHTML .= "<option value='".$roomId."'>" . $roomId . "</option>".PHP_EOL;
    }

    $roomHTML .= '</select>';