array_unique vs array_flip

If I had an array of signed integers e.g:

Array
(
    [0] => -3
    [1] => 1
    [2] => 2
    [3] => 3
    [4] => 3
)

To get unique values I would instinctively use array_unique but after consideration I could perform array_flip twice which would have the same effect, and I think it would be quicker?

array_unique O(n log n) because of the sort operation it uses

array_flip O(n)

Am I correct in my assumptions?

UPDATE / EXAMPLE:

$intArray1 = array(-4,1,2,3);
print_r($intArray1);
$intArray1 = array_flip($intArray1);
print_r($intArray1);
$intArray1 = array_flip($intArray1);
print_r($intArray1);

Array
(
    [0] => -3
    [1] => 1
    [2] => 2
    [3] => 3
    [4] => 3
)
Array
(
    [-3] => 0
    [1] => 1
    [2] => 2
    [3] => 4
)
Array
(
    [0] => -3
    [1] => 1
    [2] => 2
    [4] => 3
)