How to get only keys from a nested array in php?

{
    "numberOfResults": 1,
"items": [
    {
        "asin": "B089SMNFZJ",
        "attributes": {
            "model_name": [
                {
                    "language_tag": "en_US",
                    "value": "Arko",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "age_range_description": [
                {
                    "language_tag": "en_US",
                    "value": "Adult",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "bullet_point": [
                {
                    "language_tag": "en_US",
                    "value": "After washing with hand soap, use Arko Nem Cream give dry hands the soothing care they deserve",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Intensive Cream",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Fast absorbing",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Delivers lng lasting moisture",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "scent": [
                {
                    "language_tag": "en_US",
                    "value": "Classic",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "material_type_free": [
                {
                    "language_tag": "en_US",
                    "value": "Alcohol Free",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_dimensions": [
                {
                    "width": {
                        "unit": "centimeters",
                        "value": 10.0
                    },
                    "length": {
                        "unit": "centimeters",
                        "value": 15.0
                    },
                    "height": {
                        "unit": "centimeters",
                        "value": 10.0
                    },
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "product_description": [
                {
                    "language_tag": "en_US",
                    "value": "<br>Arko Nem<br><br>Shine like your skin!<br>Protects and vivifies your skin.<br><br>Arko Nem Classic Oily Cream<br>&nbsp;<br>This efficient moisturizer that contains the clean and nostalgic scent of classic Arko creams, is a guaranteed solution to the dryness that your skin experience in the cold of winter, wind of spring and the glowing days of the summer. Despite it has an oil-based formula, Arko Nem Classic easily absorbs and provides a comfortable use.&nbsp;<br>It has been dermatologically tested and can be used safely on children's skin.&nbsp;<br>During the day, weather changes and various other external factors cause your body the lose its natural moisture balance.&nbsp;<br><br>It moisturizes without leaving a greasy feeling on your skin and allows it to reach the natural moisture balance.<br>Its effect lasts 24 hours and meets your skin’s moisture needs all day.&nbsp;<br>300ml, 100ml, 20cc<br>With 3 sizes of 20cc, 100ml and 300ml, you can easily put Arko cream in your bag, take it to your travels and share it with your loved ones at home.<br><br>RECCOMENDATIONS FOR USE<br><br>Massage to your hands and face and apply all areas that need moisture until the cream is absorbed.&nbsp;<br>Massage and let your skin absorb the cream until it disappears.<br>Arko Nem Classic moisturizing cream is with you everywhere!&nbsp;<br>It provides your skin's natural moisture balance and does not leave a greasy feeling.&nbsp;<br><br>DISCOVER THE ARKO NEM SERIES!<br><br>SOFT TOUCH<br>GLYCERIN<br>CLASSIC<br>PEARL ESSENCES<br>OLIVE OIL&nbsp;<br><br>250ml&amp;60ml Features, benefits, areas of usage<br><br>For normal and combination skins<br>With its formula that absorbs quickly and does not leave a greasy feeling, it gives moisture to your skin.<br><br>For severe dry skins<br>It traps moisture in your hands and provides an ideal moisture balance for severe dry hands.<br><br>For dry skins",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "brand": [
                {
                    "language_tag": "en_US",
                    "value": "Arko",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "unspsc_code": [
                {
                    "value": "53130000",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_display_weight": [
                {
                    "unit": "grams",
                    "value": 200.0,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "externally_assigned_product_identifier": [
                {
                    "value": "091952669056",
                    "type": "upc",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "value": "0091952669056",
                    "type": "ean",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_form": [
                {
                    "language_tag": "en_US",
                    "value": "Cream",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_type_keyword": [
                {
                    "value": "facial-moisturizers",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "number_of_items": [
                {
                    "value": 5,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_package_dimensions": [
                {
                    "length": {
                        "unit": "centimeters",
                        "value": 3.302
                    },
                    "width": {
                        "unit": "centimeters",
                        "value": 10.16
                    },
                    "height": {
                        "unit": "centimeters",
                        "value": 11.938
                    },
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "recommended_uses_for_product": [
                {
                    "language_tag": "en_US",
                    "value": "Soothing",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "size": [
                {
                    "language_tag": "en_US",
                    "value": "Pack of 5",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "part_number": [
                {
                    "value": "504785",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "target_gender": [
                {
                    "value": "unisex",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "variation_theme": [
                {
                    "name": "SIZE_NAME/SCENT_NAME",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "contains_liquid_contents": [
                {
                    "value": true,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_package_weight": [
                {
                    "unit": "kilograms",
                    "value": 0.136,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_volume": [
                {
                    "unit": "milliliters",
                    "value": 300.0,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "skin_type": [
                {
                    "language_tag": "en_US",
                    "value": "Oily,Dry",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "manufacturer": [
                {
                    "language_tag": "en_US",
                    "value": "Evyap",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "target_use_body_part": [
                {
                    "value": "hands",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "model_number": [
                {
                    "value": "504785",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "supplier_declared_dg_hz_regulation": [
                {
                    "value": "not_applicable",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "material_feature": [
                {
                    "language_tag": "en_US",
                    "value": "Natural",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "item_name": [
                {
                    "language_tag": "en_US",
                    "value": "Arko Classic Oily Cream 20cc (5 Pack)",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "list_price": [
                {
                    "currency": "USD",
                    "value": 0.0,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "batteries_required": [
                {
                    "value": false,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "product_site_launch_date": [
                {
                    "value": "2020-06-16T07:23:07.142Z",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "target_audience_keyword": [
                {
                    "language_tag": "en_US",
                    "value": "Unisex Adult",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Men",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Adults",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "People",
                    "marketplace_id": "ATVPDKIKX0DER"
                },
                {
                    "language_tag": "en_US",
                    "value": "Teens",
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ],
            "unit_count": [
                {
                    "type": {
                        "language_tag": "en_US",
                        "value": "Count"
                    },
                    "value": 6.0,
                    "marketplace_id": "ATVPDKIKX0DER"
                }
            ]
        },
        "dimensions": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "item": {
                    "height": {
                        "unit": "inches",
                        "value": 3.93700787
                    },
                    "length": {
                        "unit": "inches",
                        "value": 5.905511805
                    },
                    "width": {
                        "unit": "inches",
                        "value": 3.93700787
                    }
                },
                "package": {
                    "height": {
                        "unit": "inches",
                        "value": 1.299999998674
                    },
                    "length": {
                        "unit": "inches",
                        "value": 4.699999995206
                    },
                    "weight": {
                        "unit": "pounds",
                        "value": 0.29982867632
                    },
                    "width": {
                        "unit": "inches",
                        "value": 3.99999999592
                    }
                }
            }
        ],
        "identifiers": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "identifiers": [
                    {
                        "identifierType": "EAN",
                        "identifier": "0091952669056"
                    },
                    {
                        "identifierType": "UPC",
                        "identifier": "091952669056"
                    }
                ]
            }
        ],
        "images": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "images": [
                    {
                        "variant": "MAIN",
                        "link": "https://m.media-amazon.com/images/I/71OWUn6w2HL.jpg",
                        "height": 1500,
                        "width": 1500
                    },
                    {
                        "variant": "MAIN",
                        "link": "https://m.media-amazon.com/images/I/51vBolkWZkL.jpg",
                        "height": 500,
                        "width": 500
                    },
                    {
                        "variant": "MAIN",
                        "link": "https://m.media-amazon.com/images/I/51vBolkWZkL._SL75_.jpg",
                        "height": 75,
                        "width": 75
                    },
                    {
                        "variant": "PT01",
                        "link": "https://m.media-amazon.com/images/I/31G-MWnA8sL.jpg",
                        "height": 432,
                        "width": 391
                    },
                    {
                        "variant": "PT01",
                        "link": "https://m.media-amazon.com/images/I/31G-MWnA8sL._SL75_.jpg",
                        "height": 75,
                        "width": 68
                    },
                    {
                        "variant": "PT02",
                        "link": "https://m.media-amazon.com/images/I/41fE9c5y2ML.jpg",
                        "height": 479,
                        "width": 494
                    },
                    {
                        "variant": "PT02",
                        "link": "https://m.media-amazon.com/images/I/41fE9c5y2ML._SL75_.jpg",
                        "height": 73,
                        "width": 75
                    },
                    {
                        "variant": "PT03",
                        "link": "https://m.media-amazon.com/images/I/31o+SLhcIpL.jpg",
                        "height": 286,
                        "width": 302
                    },
                    {
                        "variant": "PT03",
                        "link": "https://m.media-amazon.com/images/I/31o+SLhcIpL._SL75_.jpg",
                        "height": 71,
                        "width": 75
                    },
                    {
                        "variant": "PT04",
                        "link": "https://m.media-amazon.com/images/I/31VNiSOl3SL.jpg",
                        "height": 292,
                        "width": 311
                    },
                    {
                        "variant": "PT04",
                        "link": "https://m.media-amazon.com/images/I/31VNiSOl3SL._SL75_.jpg",
                        "height": 70,
                        "width": 75
                    }
                ]
            }
        ],
        "relationships": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "relationships": []
            }
        ],
        "salesRanks": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "classificationRanks": [
                    {
                        "classificationId": "16479981011",
                        "title": "Face Moisturizers",
                        "link": "https://www.amazon.com/gp/bestsellers/beauty/16479981011",
                        "rank": 4613
                    }
                ],
                "displayGroupRanks": [
                    {
                        "websiteDisplayGroup": "beauty_display_on_website",
                        "title": "Beauty & Personal Care",
                        "link": "https://www.amazon.com/gp/bestsellers/beauty",
                        "rank": 227725
                    }
                ]
            }
        ],
        "summaries": [
            {
                "marketplaceId": "ATVPDKIKX0DER",
                "adultProduct": false,
                "autographed": false,
                "brand": "Arko",
                "browseClassification": {
                    "displayName": "Face Moisturizers",
                    "classificationId": "16479981011"
                },
                "itemClassification": "BASE_PRODUCT",
                "itemName": "Arko Classic Oily Cream 20cc (5 Pack)",
                "manufacturer": "Evyap",
                "memorabilia": false,
                "modelNumber": "504785",
                "partNumber": "504785",
                "size": "Pack of 5",
                "tradeInEligible": false,
                "websiteDisplayGroup": "beauty_display_on_website",
                "websiteDisplayGroupName": "Beauty"
            }
        ]
    }
]
}

I get JSON data for ASINs from Amazon using the SP API, as above.

This data is;

$data = json_decode($DATA_SEC_TXT_1, true);

The sample output is as follows.

Array
(
[0] => Array
    (
        [asin] => B089SMNFZJ
        [attributes] => Array
            (
                [model_name] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Arko
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [age_range_description] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Adult
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [bullet_point] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => After washing with hand soap, use Arko Nem Cream give dry hands the soothing care they deserve
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                        [1] => Array
                            (
                                [language_tag] => en_US
                                [value] => Intensive Cream
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                        [2] => Array
                            (
                                [language_tag] => en_US
                                [value] => Fast absorbing
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                        [3] => Array
                            (
                                [language_tag] => en_US
                                [value] => Delivers lng lasting moisture
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [scent] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Classic
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [material_type_free] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Alcohol Free
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [item_dimensions] => Array
                    (
                        [0] => Array
                            (
                                [width] => Array
                                    (
                                        [unit] => centimeters
                                        [value] => 10
                                    )

                                [length] => Array
                                    (
                                        [unit] => centimeters
                                        [value] => 15
                                    )

                                [height] => Array
                                    (
                                        [unit] => centimeters
                                        [value] => 10
                                    )

                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [product_description] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Arko Nem
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [brand] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Arko
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [unspsc_code] => Array
                    (
                        [0] => Array
                            (
                                [value] => 53130000
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )
                [supplier_declared_dg_hz_regulation] => Array
                    (
                        [0] => Array
                            (
                                [value] => not_applicable
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [material_feature] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Natural
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [item_name] => Array
                    (
                        [0] => Array
                            (
                                [language_tag] => en_US
                                [value] => Arko Classic Oily Cream 20cc (5 Pack)
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                [list_price] => Array
                    (
                        [0] => Array
                            (
                                [currency] => USD
                                [value] => 0
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )

                                    [product_site_launch_date] => Array
                    (
                        [0] => Array
                            (
                                [value] => 2020-06-16T07:23:07.142Z
                                [marketplace_id] => ATVPDKIKX0DER
                            )

                    )
        [summaries] => Array
            (
                [0] => Array
                    (
                        [marketplaceId] => ATVPDKIKX0DER
                        [adultProduct] => 
                        [autographed] => 
                        [brand] => Arko
                        [browseClassification] => Array
                            (
                                [displayName] => Face Moisturizers
                                [classificationId] => 16479981011
                            )

                        [itemClassification] => BASE_PRODUCT
                        [itemName] => Arko Classic Oily Cream 20cc (5 Pack)
                        [manufacturer] => Evyap
                        [memorabilia] => 
                        [modelNumber] => 504785
                        [partNumber] => 504785
                        [size] => Pack of 5
                        [tradeInEligible] => 
                        [websiteDisplayGroup] => beauty_display_on_website
                        [websiteDisplayGroupName] => Beauty
                    )

            )

    )

)

I can easily process this php data with the code;

print_r ($data["items"]);

What I want to do is to get the keys of nested arrays in PHP codes.

The output I want to get should be like this.

What I want to do is to get the keys of nested arrays in PHP codes.

So every;

[0] => Array

I want to go over the previous keys.

The output I want to get is;

[asin] => B089SMNFZJ
[attributes] => Array
(
[0] => model_name
[1] => age_range_description
[0] => bullet_point
...
...

How can I do this?