Using array_diff_key() to Compute Array Differences by Keys in PHP

The array_diff_key() function compares the keys of one array against one or more other arrays and returns the entries from the first array whose keys are not present in any of the subsequent arrays. It works similarly to array_diff() but uses key comparison instead of value comparison.

Function signature : array_diff_key(array $array1, array $array2, ... $arrayN) Parameters

array1 : The array to compare against the others.

array2 , arrayN : One or more arrays whose keys will be used for comparison.

Return value

The function returns an array containing all key‑value pairs from $array1 whose keys do not appear in any of the other supplied arrays.

Example

<?php
$array1 = array(
    'blue'   => 1,
    'red'    => 2,
    'green'  => 3,
    'purple' => 4
);

$array2 = array(
    'green'  => 5,
    'blue'   => 6,
    'yellow' => 7,
    'cyan'   => 8
);

var_dump(array_diff_key($array1, $array2));
?>

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, so they are returned by array_diff_key().