Dynamic variable name for a multidimensional array

On my left hand I've got this "key": localisation.adresse.commune.id and many other values like this one, which are dynamic (I can't use them literally in my code as I don't know what they will be).

On the other hand I have an array like this (comes from a JSON decoded):

    Array
        (
            [localisation] => Array
                (
                    [adresse] => Array
                        (
                            [adresse1] => Le Chatelard
                            [codePostal] => 42820
                            [etat] => France
                            [commune] => Array
                                (
                                    [id] => 16418
                                )
                        )
                )
        )

I can't list all the "keys" I'm going to exploit, so I need to automatically get the value of $object['localisation']['adresse']['commune']['id'].

I've tried this but it does not work:

$test['localisation']['adresse']['commune']['id'] = 16418 ;
$var = '$test[\'localisation\'][\'adresse\'][\'commune\'][\'id\']' ;
echo $var ; // $test['localisation']['adresse']['commune']['id']
var_dump($$var) ; // NULL Notice: Undefined variable: $test['localisation']['adresse']['commune']['id']
var_dump(${$var}) ; // NULL Notice: Undefined variable: $test['localisation']['adresse']['commune']['id']

I suppose it's looking for a simple variable of a complicated name, instead of looking at a multidimensional array, but I don't know how I can do it...

Thanks for your help!

I see no other way except traversing the array and trying to find the keys in internal arrays, if there are any.

I came up with two variants: recursive and iterative. They will also handle the case when the "depth" of the key and array differ, e.g. if your $key will contain more elements than the depth of the array, then the NULL will be returned, if less - then whatever is under the last key will be returned.

Recursive variant

$a = [
    'localisation' => [
        'adresse' => [
            'adresse1' => 'Le Chatelard',
            'codePostal' => 42820,
            'etat' => 'France',
            'commune' => [
                'id' => 16418,
            ],
        ],
    ],
];

$key = 'localisation.adresse.commune.id';

function getValueByKeyRecursively($a, $key)
{
    $keyList = explode('.', $key);
    $currentKey = array_shift($keyList);

    // Found the value
    if (empty($currentKey)) {
        return $a;
    }

    // No more depth to traverse or no such key
    if (!is_array($a) || !array_key_exists($currentKey, $a)) {
        return null;
    }

    return getValueByKeyRecursively($a[$currentKey], implode('.', $keyList));
}

var_dump(getValueByKeyRecursively($a, $key)); // outputs: int(16418)

Iterative variant

function getValueByKey(array $a, $key)
{
    $keyList = explode('.', $key);
    $returnValue = $a;

    do {
        $currentKey = array_shift($keyList);

        // Found the value
        if ($currentKey === null) {
            break;
        }

        // No more depth to traverse or no such key
        if (!is_array($returnValue) || !array_key_exists($currentKey, $returnValue)) {
            return null;
        }

        $returnValue = $returnValue[$currentKey];

    } while (true);

    return $returnValue;
}

var_dump(getValueByKey($a, $key)); // outputs: int(16418)