Retreiving sub-array indexes

Question

Given [1=>[4=>[]],3=>[2=>[]],0=>[6=>[]],8=>[2=>[]]], how can I obtain [4,2,6]? I can obviously use a foreach() loop, but am interested whether a more concise solution is available.

<?php

$arr1=[
    1=>[4=>[]],
    3=>[2=>[]],
    0=>[6=>[]],
    8=>[2=>[]],
];
print_r($arr1);
$arr2=[4,2,6,2];
print_r($arr1);
$arr3=array_values(array_unique($arr2));
print_r($arr3);


Array
(
    [1] => Array
        (
            [4] => Array
                (
                )

        )

    [3] => Array
        (
            [2] => Array
                (
                )

        )

    [0] => Array
        (
            [6] => Array
                (
                )

        )

    [8] => Array
        (
            [2] => Array
                (
                )

        )

)
Array
(
    [1] => Array
        (
            [4] => Array
                (
                )

        )

    [3] => Array
        (
            [2] => Array
                (
                )

        )

    [0] => Array
        (
            [6] => Array
                (
                )

        )

    [8] => Array
        (
            [2] => Array
                (
                )

        )

)
Array
(
    [0] => 4
    [1] => 2
    [2] => 6
)

Answer 1

The shortest one with array_map and key functions:

$result = array_unique(array_map(function($a){ return key($a); }, $arr1));
print_r($result);

The output:

Array
(
    [1] => 4
    [3] => 2
    [0] => 6
)

Answer 2

You could probably do something like this:

$arr1=[
    1=>[4=>[]],
    3=>[2=>[]],
    0=>[6=>[]],
    8=>[2=>[]],
];
$arr2 = array_unique(array_reduce($arr1, function ($a, $b) {
    $a = array_merge($a, array_keys($b));
    return $a;
}, []));
print_r($arr2);

This is similar to the solution proposed in the first comment but this one will also work when you have more than one items in the inner arrays