How to Compute Array Differences by Key Using array_diff_ukey with a Callback in PHP

What is array_diff_ukey ? It returns an array containing the entries from the first array whose keys are not present in any of the subsequent arrays, while preserving the original key‑value associations. The comparison is performed on keys rather than values, and a user‑defined callback can control the comparison logic.

Parameters array1 – The first array to compare. array2 – One or more additional arrays to compare against array1. key_compare_func – 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.

Return value

An array containing the entries from array1 whose keys do not appear in any of the other arrays.

Example usage

<?php
function key_compare_func($key1, $key2) {
    if ($key1 == $key2) return 0;
    elseif ($key1 > $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'));
?>

The above script outputs the keys that exist only in $array1 after the comparison:

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

This demonstrates how a custom callback can be used to fine‑tune key comparison, enabling precise control over which elements are considered different.