How to flatten a PHP array

function true_flatten(array $array, array $parents = [])

{

    $return = [];

    foreach ($array as $k => $value) {

        $p = empty($parents) ? [$k] : [...$parents, $k];

        if (is_array($value)) {

            $return = [...$return, ...true_flatten($value, $p)];

        } else {

            $return[implode('_', $p)] = $value;

        }

    }



    return $return;

}

$r = [

    'a' => 'value1',

    'b' => [

        'c' => 'value1.1',

        'd' => 'value1.2',

    ],

    'e' => 'value2',

    'f' => [

        'j' => [

            'k' => 'value2.1',

            'm' => 'value2.2',

            'n' => 'value2.3',

        ],

    ],

    'o' => 'value3',

    'p' => [

        'some' => [

            'very' => [

                'deep' => [

                    'item' => [

                        'first',

                        'second',

                    ]

                ]

            ]

        ]

    ],

    'q' => 'value5',

]; // sample data

{

    "a": "value1",

    "b_c": "value1.1",

    "b_d": "value1.2",

    "e": "value2",

    "f_j_k": "value2.1",

    "f_j_m": "value2.2",

    "f_j_n": "value2.3",

    "o": "value3",

    "p_some_very_deep_item_0": "first",

    "p_some_very_deep_item_1": "second",

    "q": "value5"

}