array_diff_key()
array_diff_key() is a PHP native function that computes the difference between arrays, comparing keys instead of values.
array_diff_key() takes an array $array, and one or several other arrays ...$arrays. It returns a new array made of the elements of $array whose key is not present in any of the other arrays, keeping the original keys and values of $array.
Since only the keys are compared, and never cast to string, array_diff_key() is safe to use with arrays holding objects, arrays, or resources as values.
The complement function is array_intersect_key(), which returns the elements whose key is shared amongst the arrays. array_diff_assoc() compares both keys and values, while plain array_diff() compares values only.
<?php
$array1 = ['a' => 'green', 'b' => 'brown', 'c' => 'blue', 'red'];
$array2 = ['a' => 'yellow', 'b' => 'orange', 'green'];
$result = array_diff_key($array1, $array2);
print_r($result);
// ['c' => 'blue']
?>