Sometimes we need to partition our data, which can be done in many ways. In this this article I'll show you how to do this using PHP's array_reduce function.

First lets look at parameters that you can pass the function.

• The first parameter should always be an array.
• The second parameter is a call back function, which is where all the magic will happen.
• The third, optional, parameter is where we set the initial value.

First let's look at the basic usage by adding up an array of numbers.

You can also use array_sum, but for the purposes of this tutorial we'll use array_reduce

$numbers = [1, 3, 9, 12, 15, 18, 21];

$result = array_reduce($numbers, function($carry, $number) {
    return $carry + $number;
}, 0);

Our result would be 79 as expected.

Next we'll look at splitting up the numbers array into 2 groups. Small numbers being anything under 10, and large numbers being 10 and over.

$numbers = [1, 3, 9, 12, 15, 18, 21];

$result = [
    'small' => [],
    'large' => [],
];

$result = array_reduce($numbers, function($carry, $number) {
    if($number >= 10) {
        array_push($carry['large'], $number);   
    }

    if($number < 10) {
        array_push($carry['small'], $number);   
    }

    return $carry;
}, $result);

The output of $result would be the following as we expect:

Array
(
    [small] => Array
        (
            [0] => 1
            [1] => 3
            [2] => 9
        )

    [large] => Array
        (
            [0] => 12
            [1] => 15
            [2] => 18
            [3] => 21
        )

)

Lastly, we'll take another example of taking a group of users and splitting them up by gender.

$users = [
    ['name' => 'Frank', 'gender' => 'Male'],
    ['name' => 'Michael', 'gender' => 'Male'],
    ['name' => 'David', 'gender' => 'Male'],
    ['name' => 'Jabari', 'gender' => 'Male'],
    ['name' => 'Sarah', 'gender' => 'Female'],
    ['name' => 'Jessy', 'gender' => 'Female'],
    ['name' => 'Tracey', 'gender' => 'Female'],
];

$usersByGender = array_reduce(
    $users, 
    function($groups, $user) {
        if($user['gender'] === 'Male') {
            array_push($groups['male'], $user);
        }

        if($user['gender'] === 'Female') {
            array_push($groups['female'], $user);
        }

        return $groups;
    }, 
    [
        'male' => [],
        'female' => [],
    ]
);

As expected our output would be the following:

Array
(
    [male] => Array
        (
            [0] => Array
                (
                    [name] => Frank
                    [gender] => Male
                )

            [1] => Array
                (
                    [name] => Michael
                    [gender] => Male
                )

            [2] => Array
                (
                    [name] => David
                    [gender] => Male
                )

            [3] => Array
                (
                    [name] => Jabari
                    [gender] => Male
                )

        )

    [female] => Array
        (
            [0] => Array
                (
                    [name] => Sarah
                    [gender] => Female
                )

            [1] => Array
                (
                    [name] => Jessy
                    [gender] => Female
                )

            [2] => Array
                (
                    [name] => Tracey
                    [gender] => Female
                )

        )

)