Array Chunk

Using array chunk (array_chunk) we can easily split an array into a multidimensional array (an array of arrays).

The array chunk function could be used to break an array into smaller parts, perhaps to make it easier to process.

Basic Example
Composition
Features and Specific Examples
Further Reading

Basic Example

Assuming we setup an array as per the below code example ($array on line 1), we can chunk this array into a new multidimensional array using the array_chunk function on line 9.

Running this code would result in the multidimensional array on line 15.

$array = [
    'Post 0', 
    'Post 1', 
    'Post 2', 
    'Post 3', 
    'Post 4', 
    'Post 5',
];
$chunked_array = array_chunk( $array, 2 );
print_r( $chunked_array ); // Output the new array.

/**
 * Output:
 *
 * Array
 * (
 *   [0] => Array
 *       (
 *          [0] => Post 0
 *          [1] => Post 1
 *       )
 *   [1] => Array
 *       (
 *          [0] => Post 2
 *          [1] => Post 3
 *       )
 *   [2] => Array
 *       (
 *           [0] => Post 4
 *           [1] => Post 5
 *       )
 *)
 **/

Composition

$chunked_array = array_chunk( $array, $chunk, $preserve_keys );

The array_chunk function accepts three parameters, $array, $chunk and $preserve_keys (optional, false by default). These are as follows:

The $array parameter accepts an array, and is the initial array that we we wish to transform.
The $chunk accepts an integer, and is the number that we use to split the $array into parts of this value.
The optional $preserve_keys parameter accepts a boolean, and will preserve the keys of the array if true.

Features and Specific Examples

Preserve Keys

Exploring the $preserve_keys parameter, our previous example starts the index of each nested array at 0. However if we were to pass true into this parameter, the original index of initial array would be kept intact:

$array = [
    'Post 0', 
    'Post 1', 
    'Post 2', 
    'Post 3', 
    'Post 4', 
    'Post 5',
];
$chunked_array = array_chunk( $array, 2 , true );
print_r( $chunked_array ); // Output the new array.

/**
 * Output:
 *
 * Array
 * (
 *   [0] => Array
 *       (
 *          [0] => Post 0
 *          [1] => Post 1
 *       )
 *   [1] => Array
 *       (
 *          [2] => Post 2
 *          [3] => Post 3
 *       )
 *   [2] => Array
 *       (
 *           [4] => Post 4
 *           [5] => Post 5
 *       )
 *)
 **/

In the output the keys are now sequential, as they would be in the original array (0, 1, 2, 3…).

Preserve Keys when the Original Array has Named Keys

If our original array has named keys, then we can preserve the keys to maintain these in our chunked array.

The following example has a bit of fun with this. Let’s say we have somehow got an array with some Post ID’s and Post Names mixed together.

We could use array_chunk with the $preserve_keys parameter set to true to split these into the individual posts.

$array = [
    'id_0'   => '10', 
    'name_0' => 'Post 1', 
    'id_1'   => '20', 
    'name_1' => 'Post 2', 
    'id_2'   => '30', 
    'name_2' => 'Post 3',
];
$chunked_array = array_chunk( $array, 2 , true);
print_r( $chunked_array ); // Output the new array.

/**
 * Output:
 *
 * Array
 * (
 *   [0] => Array
 *       (
 *          [id_0] => 10
 *          [name_0] => Post 1
 *       )
 *   [1] => Array
 *       (
 *          [id_1] => 20
 *          [name_1] => Post 2
 *       )
 *   [2] => Array
 *       (
 *           [id_2] => 30
 *           [name_2] => Post 3
 *       )
 *)
 **/

Still having fun with this array, let’s imagine how we could loop through the $chunked_array, and output only the names.

foreach( $chunked_array as $key => $nested_array ) {
    echo $nested_array[ 'name_' . $key ] . PHP_EOL;
}

/**
 * Output: 
 *
 * Post 1
 * Post 2
 * Post 3
 **/

Chunking a Large Array for Processing.

If we have a very large array, we can use array_chunk to break it into smaller chunks that we could then store separately. Let’s see how we might do this in WordPress:

$original_array = [...]; // Let's imagine this is a very large array.
$post_id        = get_the_ID();
$chunked_array  = array_chunk( $original_array, 10, true );

// Let's store it as smaller parts.
foreach( $chunked_array as $key => $array ) {
    update_post_meta( 
        $post_id,
        '_wc_chunked_array_partial_' . $post_id,
        $array
    );
}