The behavior of array_diff_key() when an array key is not a string or an integer

M66 2025-05-15

In PHP, the array_diff_key() function is used to calculate the key differences between two or more arrays. It returns an array containing the differences between all keys in the first array that exist in other arrays. If the keys in both arrays are exactly the same, the result is an empty array.

 <?php
$array1 = [
    'apple' => 'fruit',
    'carrot' => 'vegetable',
    'pear' => 'fruit'
];

$array2 = [
    'apple' => 'fruit',
    'carrot' => 'vegetable'
];

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

Output:

 Array
(
    [pear] => fruit
)

This code shows how to calculate the difference between keys in two arrays by the array_diff_key() function. In this example, pearl only appears in $array1 , so it is returned.

The manifestation of a key is neither a string nor an integer

Let's further explore a more special situation: How will array_diff_key() behave when the keys of an array are neither strings nor integers? In PHP, the keys of an array can be integers, strings, or other types of values (such as objects, etc.). However, PHP converts certain data types to string or integer types as array keys. Therefore, type conversion may occur when the keys of an array are neither strings nor integers.

Consider the following code example:

 <?php
$array1 = [
    new DateTime() => 'time object',
    3.14159 => 'pi',
    true => 'boolean'
];

$array2 = [
    '2025-04-18 10:00:00' => 'time object',
    3 => 'pi',
    '1' => 'boolean'
];

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

In this example, $array1 uses different types of keys: DateTime object, floating point number, and boolean. $array2 uses the string '2025-04-18 10:00:00' , integer 3 and string '1' as keys. Let's analyze how array_diff_key() will handle these keys.

PHP When processing array keys, non-integer and non-string keys are typed. Specifically:

The DateTime object is converted to its string representation ( DateTime::__toString() ).
The floating point number 3.14159 will be converted to the string "3.14159" .
The boolean true value will be converted to the string "1" .

Therefore, array_diff_key() may be compared with the original keys when processing these keys, and the type-converted keys are considered different.

in conclusion

array_diff_key() will compare according to the key type conversion rules. If the keys of an array are neither strings nor integers, PHP will automatically convert these keys to strings or integers and compare them based on these converted values. Unintuitive results may occur for certain special data types, especially objects or booleans. Therefore, when using array_diff_key() , it is best to make sure that the keys of the array are strings or integers to avoid unexpected type conversions.

array_diff_key