Question

I have tried sorting the below array to keep the even values on top and in sorted order from asc to desc

$array = array(1,2,3,4,5,6,7,8,9,10);

I tried this to sort the array

usort($array, function($a, $b) { 
                    if ($a % 2 == 0 )
                    { 
                        return 1 ;
                    }
                    else 
                    { 
                        return -1;
                    }
            }
        );

I got the output like below

Array
(
    [0] => 7
    [1] => 9
    [2] => 1
    [3] => 5
    [4] => 3
    [5] => 2
    [6] => 4
    [7] => 6
    [8] => 8
    [9] => 10
)

And I want the output array to be

Array
(
    [0] => 2
    [1] => 4
    [2] => 6
    [3] => 8
    [4] => 10
    [5] => 1
    [6] => 3
    [7] => 5
    [8] => 7
    [9] => 9
)

The even and odd values should be sorted in asc to desc order but keep the even values on top of odd values

Answer 1

usort is not stable. The documentation states:

If two members compare as equal, their relative order in the sorted array is undefined.

So, what you can do is:

usort($array, function($a, $b) {
                if ($a % 2 == $b % 2) {
                    return intval($a - $b);
                }
                elseif ($a % 2 == 0 )
                { 
                    return -1 ;
                }
                else 
                { 
                    return 1;
                }
        }
    );

This compares the actual values if both are even or both are odd.