<?php

use Illuminate\Support\Arr;

$array1 = [
    1, 2,
    [3, 4],
    [5],
    6, 7,
];

$array2 = [
    'one' => 1,
    'two' => 2,
    [
        'one' => 3,
        'four' => 4,
    ],
    'five' => 5,
];

$flatten1 = Arr::flatten($array1);
$flatten2 = Arr::flatten($array2);

var_dump($flatten1, $flatten2);

array(7) {
[0]=>
int(1)
[1]=>
int(2)
[2]=>
int(3)
[3]=>
int(4)
[4]=>
int(5)
[5]=>
int(6)
[6]=>
int(7)
}
array(5) {
[0]=>
int(1)
[1]=>
int(2)
[2]=>
int(3)
[3]=>
int(4)
[4]=>
int(5)
}

• Laravel DI Container
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PHP 7.4, Zend 3.0, Symfony 4.3, Laravel 5.8, PHPUnit 8.0, Doctrine 2.10