What is the best way to compare PHP dates?

Question

PHP

I have an array of dates.

Array
(
    [0] => 19.09.2016

    [1] => 24.03.2017

    [2] => 02.05.2017

    [3] => 19.06.2017

    [4] => 18.09.2017

    [5] => 30.10.2017

    [6] => 18.12.2017

)

When comparing dates from an array with a certain date greater than any array value, the same error occurs - one date remains outside the sample. Those. :
$data = 03/02/2017;
strtotime($data) < strtotime(Array[1]) although the selection must capture Array[0] as well. Of course, the crutch with the addition of Array[0] did, but I would like to understand the reason for the date not being included in the selection.