array_reduce() / array_walk()

array_reduce(array, callback, initial_value);

// Applies a callback to each element of the array. The original array is modified in place
array_walk(array, callback, extra_data);

sample_array_reduce.php

<?php
// Calculate the sum of an array
$prices = [150, 80, 300, 200];
$total = array_reduce($prices, fn($carry, $item) => $carry + $item, 0);
echo $total; // 730

// Find the maximum value
$scores = [85, 92, 78, 95, 88];
$max = array_reduce($scores, fn($carry, $item) => max($carry, $item), 0);
echo $max; // 95

// Concatenate array elements into a string
$words = ['PHP', 'is', 'fun'];
$sentence = array_reduce($words, fn($carry, $item) => $carry . $item, '');
echo $sentence; // PHPisfun

// Convert an associative array to HTML option tags
$options = ['jp' => 'Japan', 'us' => 'United States', 'uk' => 'United Kingdom'];
$html = array_reduce(
    array_keys($options),
    fn($carry, $key) => $carry . "<option value=\"{$key}\">{$options[$key]}</option>\n",
    ''
);
echo $html;

// Use array_walk to modify each element of an array
$members = ['item_a', 'item_b', 'item_c'];
array_walk($members, function(&$value, $key) {
    $value = ($key + 1) . '. ' . $value; // Prepend a number
});
print_r($members);

// Use array_walk to process an associative array
$scores = ['user1' => 9000, 'user2' => 8000, 'user3' => 3500];
array_walk($scores, function(&$val, $name, $multiplier) {
    $val = (int)($val * $multiplier);
}, 50);
print_r($scores);

array_reduce.php

php array_reduce.php
730
95
PHPisfun
<option value="jp">Japan</option>
<option value="us">United States</option>
<option value="uk">United Kingdom</option>
Array
(
    [0] => 1. item_a
    [1] => 2. item_b
    [2] => 3. item_c
)
Array
(
    [user1] => 450000
    [user2] => 400000
    [user3] => 175000
)

array_reduce() combines all elements of an array into a single value. The callback receives the result of the previous call as its first argument and the current element as its second. The third argument sets the initial value used for the first call; if omitted, it defaults to null.

array_walk() applies a callback to each element of the array. If you pass the first argument of the callback by reference (&$value), you can modify the original array directly. You can also pass extra data as the third argument, which is useful when you need a shared parameter — such as a tax rate — across all elements.

For simple totals or element counts, array_sum() / count() is more concise. To transform each element and produce a new array, array_map() is an option.

If you omit the third argument of array_reduce(), the initial value defaults to null. For numeric calculations, explicitly pass 0; for string concatenation, pass ''.

<?php
$scores = ['user1' => 9000, 'user2' => 8000, 'user3' => 100];

// Omitting the initial value — $carry is null on the first call
$total_wrong = array_reduce(array_values($scores), fn($carry, $item) => $carry + $item);
echo $total_wrong; // 17100 (PHP treats null as 0 in arithmetic)

// Explicitly passing the initial value (safe)
$total_safe = array_reduce(array_values($scores), fn($carry, $item) => $carry + $item, 0);
echo $total_safe; // 17100

reduce_initial.php

php reduce_initial.php
17100
17100

For numeric calculations the result may coincidentally match, but when the initial type matters — such as string concatenation or building an array — omitting it is risky.

array_walk() discards the return value of the callback. To modify the array elements, you must pass the first argument by reference using &$value.

NG pattern (returning a value has no effect on the array):

<?php
$items = ['product_a', 'product_b', 'product_c', 'product_d'];

array_walk($items, function($value, $key) {
    return strtoupper($value); // return value is ignored
});
print_r($items); // unchanged

walk_ng.php

php walk_ng.php
Array
(
    [0] => product_a
    [1] => product_b
    [2] => product_c
    [3] => product_d
)

Modify via reference (correct approach):

<?php
$items = ['product_a', 'product_b', 'product_c', 'product_d'];

array_walk($items, function(&$value, $key) {
    $value = strtoupper($value);
});
print_r($items);

walk_ok.php

php walk_ok.php
Array
(
    [0] => PRODUCT_A
    [1] => PRODUCT_B
    [2] => PRODUCT_C
    [3] => PRODUCT_D
)

When you want to transform each element and return a new array, array_map() is more appropriate than array_walk().

If you pass an empty array, array_reduce() returns the initial value unchanged. This can be used to set a default value.

<?php
// array_reduce on an empty array
$empty = [];
$result = array_reduce($empty, fn($carry, $item) => $carry + $item, 0);
echo $result; // 0 (initial value returned as-is)

// Using it to provide a default label
$names = [];
$label = array_reduce($names, fn($carry, $item) => $carry . $item . ', ', '(none)');
echo $label; // (none)

reduce_empty.php

php reduce_empty.php
0
(none)

Cart total calculation

Using array_reduce() to aggregate the total price from an array of product objects.

<?php
$cart = [
    ['name' => 'product_a', 'price' => 300, 'qty' => 3],
    ['name' => 'product_b', 'price' => 12000, 'qty' => 1],
    ['name' => 'product_c', 'price' => 5000, 'qty' => 2],
];

$total = array_reduce($cart, function($carry, $item) {
    return $carry + $item['price'] * $item['qty'];
}, 0);

echo 'Total: ' . number_format($total) . ' yen';

reduce_cart.php

php reduce_cart.php
Total: 22,900 yen

Grouped aggregation

Using array_reduce() to build an associative array that tallies values by name.

<?php
$results = [
    ['item' => 'product_a', 'sold' => true],
    ['item' => 'product_b', 'sold' => false],
    ['item' => 'product_a', 'sold' => true],
    ['item' => 'product_b', 'sold' => true],
    ['item' => 'product_a', 'sold' => false],
];

$sales = array_reduce($results, function($carry, $item) {
    if ($item['sold']) {
        $carry[$item['item']] = ($carry[$item['item']] ?? 0) + 1;
    }
    return $carry;
}, []);

print_r($sales);

reduce_group.php

php reduce_group.php
Array
(
    [product_a] => 2
    [product_b] => 1
)