按顺序合并两组数组键

I have multiple arrays and I'd like to get an array of just the keys, but ensure that the keys are kept in order.

The outcome of this project is that every element gets put in a row in a CSV. Now I want to get the label array as the first row in the sheet and each line should follow afterwards. Some arrays of data will have more detail, note Label 3.x

$aArr1 = array (
    'Label 1' => 'Value',
    'Label 2' => 'Value',
    'Label 3.1' => 'Value',
    'Label 4' => 'Value',
    'Label 5' => 'Value',
    'Label 6' => 'Value'
);

$aArr2 = array (
    'Label 1' => 'Value',
    'Label 2' => 'Value',
    'Label 3.1' => 'Value',
    'Label 3.2' => 'Value',
    'Label 3.3' => 'Value',
    'Label 4' => 'Value',
    'Label 5' => 'Value'
);

$aArr3 = array (
    'Label 1' => 'Value',
    'Label 2' => 'Value',
    'Label 3.1' => 'Value',
    'Label 3.2' => 'Value'
)

$aDesiredLabelArr = array (
    'Label 1',
    'Label 2',
    'Label 3.1',
    'Label 3.2',
    'Label 3.3',
    'Label 4',
    'Label 5',
    'Label 6'
);

My inputs are $aArr1, $aArr2 & $aArr3. My desired output is $aDesiredLabelArr. I'm writing in PHP.

Thanks!

$result = array_keys(array_merge($aArr1, $aArr2, $aArr3)); 
sort($result, SORT_REGULAR );


[
    (int) 0 => 'Label 1',
    (int) 1 => 'Label 2',
    (int) 2 => 'Label 3.1',
    (int) 3 => 'Label 3.2',
    (int) 4 => 'Label 3.3',
    (int) 5 => 'Label 4',
    (int) 6 => 'Label 5',
    (int) 7 => 'Label 6'
]