Using array_diff_ukey() with a Callback Function to Compute Array Differences by Keys in PHP

The array_diff_ukey() function returns an array containing the entries from $array1 whose keys are not present in any of the other arrays passed to the function. Unlike array_diff() , the comparison is performed on keys rather than values, and a user‑provided callback determines the ordering of the keys.

Parameters

array1 (required): The first array to compare.

array2 (required): The second array to compare against array1 . Additional arrays can be supplied as further arguments.

key_compare_func (required): A callable that receives two keys and must return an integer less than, equal to, or greater than zero when the first key is considered respectively less than, equal to, or greater than the second key.

Return value

The function returns an array containing all key‑value pairs from array1 whose keys are not found in any of the other arrays after being evaluated by the callback.

Example

$key2) {
        return 1;
    } else {
        return -1;
    }
}

$array1 = array('blue' => 1, 'red' => 2, 'green' => 3, 'purple' => 4);
$array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan' => 8);

var_dump(array_diff_ukey($array1, $array2, 'key_compare_func'));
?>

Output

array(2) {
  ["red"]=> int(2)
  ["purple"]=> int(4)
}

This output shows that the keys red and purple exist in $array1 but not in $array2 , demonstrating how array_diff_ukey() isolates differing keys using the custom comparison function.