问要填充dropdowns的数组，键不工作

EN

$getDisplays = "
SELECT *
FROM locations l
inner join displays d
ON d.location_id = l.id;";

$displayResult = $mysqlConn->query($getDisplays);

$displayNames = array();
foreach($displayResult as $subArray) {
  if(!array_key_exists($subArray['location_name'], $displayNames)) {
    $displayNames[$subArray['location_name']] = array();

  }
    $displayNames[$subArray['location_name']][] = $subArray['display_name'];
}

<label for="plantSelect">Select A Location</label>
  <select class="form-control" id="plantSelect">
    <?php foreach($displayNames as $displayName):?>
    <option><?php echo key($displayName)?></option>
    <?php endforeach;?>
  </select>

Array
(
[Office 1] => Array
    (
        [0] => lobby
        [1] => break room
    )

[Office 2] => Array
    (
        [0] => lobby
        [1] => break room
    )

[Office 3] => Array
    (
        [0] => Main Conference Room
        [1] => Common Area
    )

[Office 4] => Array
    (
        [0] => Break Room
        [1] => Lobby
        [2] => Line 2
    )

[Office 5] => Array
    (
        [0] => Break Room
        [1] => Lobby
        [2] => Line 1
    )

)