PHP array_diff_key compares arrays and removes elements that share the same key. It checks only keys and keeps the values of the first array.

Understand the array_diff_key

The array_diff_key function compares the keys of arrays. It returns the first array without elements that exist in other arrays.

The syntax looks like this:

array_diff_key(array $array1, array $array2, array ...$arrays)

• array1: the base array.
• array2: one array for comparison.
• …arrays: more arrays for comparison.

This is the correct syntax to follow.

The function checks the keys in the first array. If a key also exists in another array, the function removes that pair. The result has only keys that do not appear in other arrays.

Examples of the array_diff_key in PHP

Basic key comparison:

$a = ["x" => 1, "y" => 2, "z" => 3];
$b = ["y" => 9, "z" => 8];
$result = array_diff_key($a, $b);
print_r($result);

This example shows that the function removes “y” and “z” because they exist as keys in the second array. Here is the output:

Array
(
    [x] => 1
)

Use with three arrays:

$a = ["id" => 10, "name" => "Ali", "role" => "admin"];
$b = ["role" => "guest"];
$c = ["id" => 20];
$result = array_diff_key($a, $b, $c);
print_r($result);

This example checks the first array against two other arrays. The result removes “id” and “role” because they exist in $b and $c. The output:

Array
(
    [name] => Ali
)

Compare numeric keys:

$a = [10 => "apple", 20 => "pear", 30 => "grape"];
$b = [20 => "banana", 40 => "orange"];
$result = array_diff_key($a, $b);
print_r($result);

This example uses numeric keys. The function removes the key 20 because it exists in the second array. The output:

Array
(
    [10] => apple
    [30] => grape
)

Advanced case with mixed keys:

$a = ["id" => 101, 1 => "data", "flag" => true];
$b = ["id" => 500, 1 => "info"];
$c = ["status" => "ok", "flag" => false];
$result = array_diff_key($a, $b, $c);
print_r($result);

This example combines string keys and number keys. The function removes “id”, 1, and “flag” because they exist in other arrays. The output is an empty array.

Wrapping Up

You learned what array_diff_key does and how it compares keys in arrays. Here is a quick recap:

• It compares only keys.
• It removes elements with keys that appear in other arrays.
• It keeps the original values for unique keys.
• It works with string keys and numeric keys.

FAQs

$array1 = array("a" => "red", "b" => "green", "c" => "blue");
$array2 = array("a" => "yellow", "d" => "black");

$result = array_diff_key($array1, $array2);
print_r($result);

Array
(
    [b] => green
    [c] => blue
)

$array1 = array("a" => "red", "b" => "green");
$array2 = array("a" => "blue", "c" => "green");

print_r(array_diff($array1, $array2));
print_r(array_diff_key($array1, $array2));

Array
(
    [b] => green
)
Array
(
    [b] => green
)

$array1 = array("a" => "red", "b" => "green", "c" => "blue");
$array2 = array("a" => "yellow", "d" => "black");
$array3 = array("c" => "white");

$result = array_diff_key($array1, $array2, $array3);
print_r($result);

Array
(
    [b] => green
)

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