You use array_reduce in PHP to process an array and reduce it to a single value. 1. It is ideal for summing values, such as totaling an array of numbers. 2. It can build custom strings, like joining array elements with commas and "and". 3. It helps in grouping or transforming data, for example, reorganizing an array of associative arrays by a specific key. To use it effectively, start with a clear initial value, keep the callback logic simple, and apply it when maintaining state across iterations is needed.

You use array_reduce in PHP when you want to process an array and "reduce" it down to a single value. That could be a sum, a concatenated string, a filtered array—really anything you need, as long as it results in one value.

Here’s how it works at a basic level:
You pass it an array, a callback function, and optionally an initial value. The callback runs for each element in the array, carrying forward a "carry" value that gets updated each time.

Let’s break it down into real-world uses and how to apply them.

Summing values in an array

This is probably the most straightforward example. If you have an array of numbers and want to get the total, array_reduce can do that cleanly.

$numbers = [10, 20, 30, 40];
$total = array_reduce($numbers, function($carry, $item) {
    return $carry   $item;
}, 0);

• $carry starts at 0 (the initial value we provided).
• Each time through, the current item is added to $carry.
• At the end, $total will be 100.

If you skip the initial value, it'll start with null, which might not be what you want if you're expecting a number.

Building a custom string from array elements

Another common use case is turning an array into a formatted string. For example, joining names with commas and "and" before the last one.

$names = ['Alice', 'Bob', 'Charlie'];
$result = array_reduce($names, function($carry, $name) {
    if ($carry === null) {
        return $name;
    }
    return "$carry, $name";
});

That gives you "Alice, Bob, Charlie".

From there, if you wanted to make it more natural like "Alice, Bob and Charlie", you could adjust the logic inside the callback—maybe check the index or count remaining items.

Grouping or transforming data

It's not just for simple types like numbers or strings. You can also use array_reduce to restructure or filter arrays.

Say you have an array of associative arrays:

$users = [
    ['id' => 1, 'role' => 'admin'],
    ['id' => 2, 'role' => 'user'],
    ['id' => 3, 'role' => 'admin'],
];

And you want to group them by role:

$grouped = array_reduce($users, function($carry, $user) {
    $role = $user['role'];
    if (!isset($carry[$role])) {
        $carry[$role] = [];
    }
    $carry[$role][] = $user;
    return $carry;
}, []);

Now you’ll have:

[
    'admin' => [ /* user 1 and 3 */ ],
    'user' => [ /* user 2 */ ]
]

This kind of grouping is super flexible and useful when dealing with API responses or database results.

A few tips for using array_reduce

• Start with a solid initial value – It makes your logic simpler and less error-prone.
• Don’t overcomplicate the callback – Keep it focused on one task.
• Use it when you need to keep track of state – Like totals, accumulators, or conditional building.
• Consider readability – Sometimes a foreach loop is clearer, especially for complex logic.

It’s powerful, but like any tool, it’s best used when it fits the job.

So yeah, array_reduce isn’t the flashiest PHP function, but once you get comfortable with it, it opens up a lot of possibilities for processing arrays in a clean, functional way.

The above is the detailed content of how to use array_reduce on a php array. For more information, please follow other related articles on the PHP Chinese website!