function array_multidim_set($arr, $keys, $value){
    switch(count($keys)){
        case 1:
           $arr[$keys[0]] = $value;
           break;
        case 2:
           $arr[$keys[0]][$keys[1]] = $value;
           break;
        case 3:
           $arr[$keys[0]][$keys[1]][$keys[2]] = $value;
           break;

        ...and so on...

    }

    return $arr;
}

function array_multidim_set(&$arr, $keys, $value){
    $rv = &$arr;
    foreach($keys as $pk)
    {
        $rv = &$rv[$pk]; // Unused reference [ex. $rv['key1'] then $rv['key1']['key2'] .. so on ] - actually assigned to $target by reference
    }
    $rv = $value;
}

$target = ['test' => 'test'];

array_multidim_set($target, ['key1', 'key2', 'key3'], 'foo');
echo '<pre>';
print_r($target);

Array
(
    [test] => ttt
    [key1] => Array
        (
            [key2] => Array
                (
                    [key3] => foo
                )

        )

)