Get the code in an Array

It does not work for you because what is in the key currencies is an array:

"currencies": [
        {
            "code": "AFN",
            "name": "Afghan afghani",
            "symbol": "؋"
        }
    ],

In the JSON notation, when data appears between [] means that this key must be accessed as an array.

These would be the data that is in the position 0 and in this case unique of the array:

        {
            "code": "AFN",
            "name": "Afghan afghani",
            "symbol": "؋"
        }

Knowing this we can then access code doing something like: objeto.currencies[0].code .

Let's see:

In Javascript:

• You must create a JSON object from the string using JSON.parse
• then search in: currencies[0].code

var str='{
        "name": "Afghanistan",
        "topLevelDomain": [
            ".af"
        ],
        "alpha2Code": "AF",
        "alpha3Code": "AFG",
        "callingCodes": [
            "93"
        ],
        "capital": "Kabul",
        "altSpellings": [
            "AF",
            "Afġānistān"
        ],
        "region": "Asia",
        "subregion": "Southern Asia",
        "population": 27657145,
        "latlng": [
            33,
            65
        ],
        "demonym": "Afghan",
        "area": 652230,
        "gini": 27.8,
        "timezones": [
            "UTC+04:30"
        ],
        "borders": [
            "IRN",
            "PAK",
            "TKM",
            "UZB",
            "TJK",
            "CHN"
        ],
        "nativeName": "افغانستان",
        "numericCode": "004",
        "currencies": [
            {
                "code": "AFN",
                "name": "Afghan afghani",
                "symbol": "؋"
            }
        ],
        "languages": [
            {
                "iso639_1": "ps",
                "iso639_2": "pus",
                "name": "Pashto",
                "nativeName": "پښتو"
            },
            {
                "iso639_1": "uz",
                "iso639_2": "uzb",
                "name": "Uzbek",
                "nativeName": "Oʻzbek"
            },
            {
                "iso639_1": "tk",
                "iso639_2": "tuk",
                "name": "Turkmen",
                "nativeName": "Türkmen"
            }
        ],
        "translations": {
            "de": "Afghanistan",
            "es": "Afganistán",
            "fr": "Afghanistan",
            "ja": "アフガニスタン",
            "it": "Afghanistan",
            "br": "Afeganistão",
            "pt": "Afeganistão",
            "nl": "Afghanistan",
            "hr": "Afganistan",
            "fa": "افغانستان"
        },
        "flag": "https://restcountries.eu/data/afg.svg",
        "regionalBlocs": [
            {
                "acronym": "SAARC",
                "name": "South Asian Association for Regional Cooperation",
                "otherAcronyms": [],
                "otherNames": []
            }
        ],
        "cioc": "AFG"
    }';
    var json=JSON.parse(str);
    console.log(json.currencies[0].code);

In PHP

The chain may need cleaning before converting it to a JSON object:

$json=json_decode(html_entity_decode( stripslashes ($str)));

And then you access the value like this:

echo $json->currencies[0]->code

Complete test:

 $str='{
    "name": "Afghanistan",
    "topLevelDomain": [
        ".af"
    ],
    "alpha2Code": "AF",
    "alpha3Code": "AFG",
    "callingCodes": [
        "93"
    ],
    "capital": "Kabul",
    "altSpellings": [
        "AF",
        "Afġānistān"
    ],
    "region": "Asia",
    "subregion": "Southern Asia",
    "population": 27657145,
    "latlng": [
        33,
        65
    ],
    "demonym": "Afghan",
    "area": 652230,
    "gini": 27.8,
    "timezones": [
        "UTC+04:30"
    ],
    "borders": [
        "IRN",
        "PAK",
        "TKM",
        "UZB",
        "TJK",
        "CHN"
    ],
    "nativeName": "افغانستان",
    "numericCode": "004",
    "currencies": [
        {
            "code": "AFN",
            "name": "Afghan afghani",
            "symbol": "؋"
        }
    ],
    "languages": [
        {
            "iso639_1": "ps",
            "iso639_2": "pus",
            "name": "Pashto",
            "nativeName": "پښتو"
        },
        {
            "iso639_1": "uz",
            "iso639_2": "uzb",
            "name": "Uzbek",
            "nativeName": "Oʻzbek"
        },
        {
            "iso639_1": "tk",
            "iso639_2": "tuk",
            "name": "Turkmen",
            "nativeName": "Türkmen"
        }
    ],
    "translations": {
        "de": "Afghanistan",
        "es": "Afganistán",
        "fr": "Afghanistan",
        "ja": "アフガニスタン",
        "it": "Afghanistan",
        "br": "Afeganistão",
        "pt": "Afeganistão",
        "nl": "Afghanistan",
        "hr": "Afganistan",
        "fa": "افغانستان"
    },
    "flag": "https://restcountries.eu/data/afg.svg",
    "regionalBlocs": [
        {
            "acronym": "SAARC",
            "name": "South Asian Association for Regional Cooperation",
            "otherAcronyms": [],
            "otherNames": []
        }
    ],
    "cioc": "AFG"
}';

$json=json_decode(html_entity_decode( stripslashes ($str)));
echo $json->currencies[0]->code;

Exit:

AFN