array_reduce() for calculating totals

Question

Below is a snippet that uses array_reduce() to calculate totals from a detailed dataset. The data shown is representative of a larger dataset.

This code outputs the expected results. I'm seeking some peer input on how this array_reduce() is written because this is my first time using it in PHP. I'm more familiar with JavaScript .reduce() function. Is there a more concise way to write this?

<?php
$data = [
    'header' => [
        'customerId' => 28449,
        'locationId' => 1278,
        'orderId' => 764,
        'orderDate' => '2022-02-01'
    ],
    'detail' => [
        '0' => [
            'itemId' => 210711,
            'orderQuantity' => 10,
            'fillQuantity' => 8,
            'unitPrice' => 120.54
        ],
        '1' => [
            'itemId' => 582284,
            'orderQuantity' => 151,
            'fillQuantity' => 144,
            'unitPrice' => 85.68
        ],
        '2' => [
            'itemId' => 476537,
            'orderQuantity' => 87,
            'fillQuantity' => 87,
            'unitPrice' => 25.75
        ]
    ]
];

$initialValues = array_fill_keys(['itemCount', 'orderQuantity', 'fillQuantity'], 0);
$totals = [];
$totals = array_reduce($data['detail'], function ($result, $item) {
    $result['itemCount']++;
    $result['orderQuantity'] += $item['orderQuantity'];
    $result['fillQuantity'] += $item['fillQuantity'];
    return $result;
}, $initialValues);


echo '<pre>' . print_r(['data' => $data, 'totals' => $totals], 1) . '</pre>';
?>

Here is the 'totals' output, which is correct:

[totals] => Array
    (
         [itemCount] => 3
         [orderQuantity] => 248
         [fillQuantity] => 239
    )

Answer 1

I don't see anything to improve inside the callback body.

I do prefer to eliminate single-only variable declarations unless the declaration improves readability or breaks up excessively wide lines of code.

From PHP7.4, concise "arrow function syntax" (fn() => ...) is available, but it cannot be used with array_reduce() because the syntax does not allow multiline bodies. (Although there is a proposal to accommodate multiline bodies with less syntax -- PHP RFC: Auto-capturing multi-statement closures)

If you are going to write function parameters on multiple lines, then write all parameters on separate lines for easier (human) reading.

Declaring $total as an empty array then immediately and unconditionally overwriting it is not needed -- just omit the empty declaration.

Suggested code:

$totals = array_reduce(
    $data['detail'],
    function ($result, $item) {
        ++$result['itemCount'];
        $result['orderQuantity'] += $item['orderQuantity'];
        $result['fillQuantity'] += $item['fillQuantity'];
        return $result;
    },
    array_fill_keys(['itemCount', 'orderQuantity', 'fillQuantity'], 0)
);