Save difference of two arrays in another array in PHP

Question

I'm trying to save the difference of two arrays in a new one.

I've used array_diff and the code I have right now is partially working but it seems that the new array saves the elements in wrong position.

<?php 
    // Arrays to hold the numbers for this demo
    $arr1 = array(5,6,7,8);
    $arr2 = array(2,5,6);
    $arr3 = array_diff($arr1, $arr2);

    // Correct result but in wrong positions in the array
    var_dump($arr3); // array(2) { [2]=> int(7) [3]=> int(8) }
?>

How is it possible to have a value at position 2 and 3 of an array of size 2?

Any explanation on the cause of this issue is greatly appreciated, thank you.

`How is it possible to have a value at position 2 and 3 of an array of size` - because that is their position in `$arr1` - `[0=>5,1=>6,2=>7,3=>8]` - array diff doesn't change that, it returns all items from array1 that are not present in any of the other array inputs. There is no rule saying arrays must start at `0`, and indeed in most cases it's better to have the original keys. The cases where it's not better, mainly it doesn't matter what the keys are. — ArtisticPhoenix, Mar 24 '19 at 20:53

score 3 · Accepted Answer · answered Mar 24 '19 at 20:50

Because arrays don't have to start at zero. As you can see, array_diff() will preserve keys.

If you want to reset the array keys you can use array_values():

// Arrays to hold the numbers for this demo
$arr1 = array(5,6,7,8);
$arr2 = array(2,5,6);
$arr3 = array_values(array_diff($arr1, $arr2));
var_dump($arr3);

Output:

array(2) {
  [0]=>
  int(7)
  [1]=>
  int(8)
}

Demo

Save difference of two arrays in another array in PHP

1 Answers1