What is the behavior of array_diff_key() when the array key is null?

M66 2025-06-06

In PHP, the array_diff_key() function is used to calculate the key differences between two or more arrays. It compares the keys of two arrays and returns the keys and their values in the first array but not in the other arrays.

Normally, we pass two arrays to array_diff_key() , which compares based on the key. But, if the key of the array is null , how will this function handle it? Next we will explore this issue through code examples.

1. Basic usage of array_diff_key() function

First, let's review the basic syntax of the array_diff_key() function:

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

$array1 is the first array, and the function will look for all keys that are not in other arrays from this array.
$array2, ...$arrays are one or more arrays, which are used by array_diff_key() to compare key values.

2. The key is null

Let's take a look at how array_diff_key() will behave when the key of the array is null . Consider the following example:

 <?php
$array1 = [
    null => 'value1',
    'key2' => 'value2',
    'key3' => 'value3'
];

$array2 = [
    null => 'value4',
    'key2' => 'value5'
];

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

print_r($result);
?>

In this example, we have two arrays $array1 and $array2 . Among them, both arrays contain an element with a key null . After running the code, the result is as follows:

 Array
(
    [key3] => value3
)

3. Explanation

As you can see from the result, array_diff_key() does not delete the element with the key null . This is because PHP will handle null as a unique key. Therefore, although both arrays contain elements with key null , they are still considered different.

To sum up, array_diff_key() treats null as a normal key and does not automatically ignore it. If there are elements with null keys in the arrays compared, they are treated as the same keys and therefore do not appear in the difference array. If a null key in an array does not exist in another array, it will be treated as a different key.

4. Application in real-life scenarios

In actual development, it is rare to see the key null , but in some special scenarios, null may be used as the default key or placeholder key. If you encounter this in your code, it is very important to understand the behavior of array_diff_key() , especially when processing data, to avoid unexpected results from wrong array key comparisons.

5. URL example

If your code involves URLs and you need to replace all domain names with m66.net , here is a simple code example:

 <?php
$url = "https://www.example.com/path/to/resource";

$new_url = preg_replace('/https?:\/\/[^\/]+/', 'https://m66.net', $url);

echo $new_url;
?>

This code replaces the domain name part in the URL with m66.net to ensure that your domain name is always consistent.

Through this article, you should have a clearer understanding of how the array_diff_key() function handles the situation where the key is null , and you have mastered how to replace the URL domain name in the code.

Hope this article helps you! If you have more questions, feel free to ask.

array_diff_key