PHP array_diff_key Function

last modified March 13, 2025

The PHP array_diff_key function compares array keys and returns the differences. It's useful for finding keys that exist in one array but not others.

Basic Definition

The array_diff_key function compares keys of multiple arrays. It returns an array containing all entries from the first array whose keys are not present in any of the other arrays.

Syntax: array_diff_key(array $array1, array ...$arrays): array. The comparison is based on keys only, not values. Key types must match.

Basic array_diff_key Example

This shows a simple comparison between two arrays with different keys.

basic_array_diff_key.php

<?php

$array1 = ['a' => 1, 'b' => 2, 'c' => 3];
$array2 = ['a' => 4, 'd' => 5];

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

print_r($result);

Output: Array ( [b] => 2 [c] => 3 ). The function returns elements from $array1 whose keys ('b' and 'c') don't exist in $array2.

Comparing Multiple Arrays

You can compare the first array against several other arrays at once.

multiple_arrays.php

<?php

$array1 = ['red' => '#FF0000', 'green' => '#00FF00', 'blue' => '#0000FF'];
$array2 = ['red' => '#FF0000', 'yellow' => '#FFFF00'];
$array3 = ['green' => '#00FF00', 'cyan' => '#00FFFF'];

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

print_r($result);

Output: Array ( [blue] => #0000FF ). Only 'blue' key exists in $array1 but not in $array2 or $array3. Values are irrelevant.

Numeric Key Comparison

The function works with numeric keys just like with string keys.

numeric_keys.php

<?php

$array1 = [10 => 'A', 20 => 'B', 30 => 'C'];
$array2 = [10 => 'X', 40 => 'Y'];

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

print_r($result);

Output: Array ( [20] => B [30] => C ). The keys 20 and 30 from $array1 don't exist in $array2, so their elements are returned.

Mixed Key Types

The function distinguishes between different key types (string vs integer).

mixed_key_types.php

<?php

$array1 = ['10' => 'String key', 10 => 'Integer key', '20' => 'Twenty'];
$array2 = [10 => 'Integer value'];

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

print_r($result);

Output: Array ( [10] => String key [20] => Twenty ). The string key '10' is different from integer key 10, so it's included.

Empty Array Behavior

When comparing with empty arrays, all keys from the first array are returned.

empty_array.php

<?php

$array1 = ['a' => 1, 'b' => 2];
$array2 = [];

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

print_r($result);

Output: Array ( [a] => 1 [b] => 2 ). Since $array2 has no keys, all keys from $array1 are considered different and returned.

Best Practices

Key Consistency: Maintain consistent key types for reliable comparisons.
Multiple Arrays: Compare against several arrays at once for efficiency.
Type Awareness: Remember that '1' and 1 are different keys.
Performance: For large arrays, consider key extraction first.

Source

PHP array_diff_key Documentation

This tutorial covered the PHP array_diff_key function with practical examples showing its usage for array key comparison scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

List all PHP Array Functions.

Ebooks