Detailed explanation of the preserve_keys parameter of array_chunk

M66 2025-04-28

What exactly does the preserve_keys parameter of the array_chunk function in PHP do? How to use it to preserve array key values?

In PHP, the array_chunk function is a commonly used function that is used to split a large array into multiple small arrays. The basic syntax of this function is as follows:

 array_chunk(array $array, int $size, bool $preserve_keys = false): array

Parameter analysis

$array : The array to be divided.
$size : The size of each subarray.
$preserve_keys : A boolean value indicating whether the key value of the original array is retained.

Here, the most commonly used parameters are $size and $preserve_keys . This article will focus on explaining the function of the $preserve_keys parameter and how to use it correctly.

The function of array_chunk function

The array_chunk function can divide an array into multiple small arrays, and the size of each small array is determined by $size . By default, the split array will use a new key value and will be renumbered from 0. If you need to preserve the key value of the original array, you can use the preserve_keys parameter to achieve it.

The function of the $preserve_keys parameter

$preserve_keys is a Boolean parameter. By default, its value is false , which means that the split subarray will lose the key value of the original array and start again from 0. If $preserve_keys is set to true , the key value of the original array will be retained.

Example 1: No key value reserved

 $array = [
    10 => 'a',
    20 => 'b',
    30 => 'c',
    40 => 'd',
    50 => 'e',
];

$result = array_chunk($array, 2);  // Key values are not retained by default

print_r($result);

Output:

 Array
(
    [0] => Array
        (
            [0] => a
            [1] => b
        )

    [1] => Array
        (
            [0] => c
            [1] => d
        )

    [2] => Array
        (
            [0] => e
        )
)

As you can see, the split array reassigns key values starting from 0 to each subarray.

Example 2: Reserve key values

 $array = [
    10 => 'a',
    20 => 'b',
    30 => 'c',
    40 => 'd',
    50 => 'e',
];

$result = array_chunk($array, 2, true);  // set uppreserve_keysfortrue，Reserve key values

print_r($result);

Output:

 Array
(
    [0] => Array
        (
            [10] => a
            [20] => b
        )

    [1] => Array
        (
            [30] => c
            [40] => d
        )

    [2] => Array
        (
            [50] => e
        )
)

At this time, the divided subarray retains the key value of the original array.

How to use preserve_keys to preserve array key values

When using the preserve_keys parameter, you only need to set it to true when calling array_chunk to preserve the key value of the original array. For example, suppose you have an array where key values make sense and you want to keep those key values, you can do this:

 $array = [
    0 => 'apple',
    1 => 'banana',
    2 => 'cherry',
    3 => 'date',
];

$chunks = array_chunk($array, 2, true);

foreach ($chunks as $chunk) {
    foreach ($chunk as $key => $value) {
        echo "Key: $key, Value: $value\n";
    }
}

Output:

 Key: 0, Value: apple
Key: 1, Value: banana
Key: 2, Value: cherry
Key: 3, Value: date

As you can see, the split array still retains the key value of the original array.

Summarize

Through the preserve_keys parameter of the array_chunk function, we can easily control whether to retain the key value of the original array. By default, key values are lost and renumbered from 0; but if you want to keep the original key values, just set preserve_keys to true .

This is very useful for scenarios where key values need to be retained, such as maintaining the structure and key-value relationship of the original array when splitting some index values or associating arrays.

Notice:

array_chunk