Retrieve value from multidimensional array using recursive keys

Question

Let's say I have a multidimensional array like this :

$array = array(
    'first' => array(
        'second' => array(
            'last1' => array(
                'key1' => 'value1',
                'key2'  => 'value2',
            ),
            'last2' => array(
                'key3' => 'value3',
                'key4' => 'value4',
            ),
        ),
    ),
);

To retrieve the value of the key key1 of the sub array last1, I would use:

$value = $array['first']['second']['last1']['key1'];
// Output value1

In my real use case, I need to retrieve the value of key1 from the following string:

$param = 'first.second.last1.key1';

I convert it to an array of keys in the good order:

$keys = explode('.', $param);
// Output array('first', 'second', 'last1', 'key1')

To retrieve the value after exploding the keys, I use the following method:

function get_value_from_keys(array $array, array $keys) {
    $lastKey = $keys[count($keys) - 1];

    foreach ($keys as $key) {
        $newArray = $newArray[$key];

        if ($key == $lastKey) {
            break;
        }
    }

    return $newArray;
}

get_value_from_keys($array, $keys);
// Output value1

But now, I need to set the value, also the question is:

How can I change the value of key1 dynamically, without hard-coding the keys ?

I cannot use the same logic as get_value_from_keys without squashing the array during the loop.