getval.php

<?
function findKey($arr, $key) {
	return $arr[$key];
}
function superFunction($arr, $path) {
	if (is_array($arr)) {
		$path_str = explode('.', $path);
		foreach ($path_str as $key) {
			$arr = findKey($arr, $key);
		}
		return $arr;
	}
}

$arr = [
	'list' => [
		'first'  => [
			'val1' => 'one',
			'val2' => 'two',
		],
		'second' => [
			'val11' => 'one1',
			'val21' => 'two2',
		],
	],
];

echo superFunction($arr, 'list.first.val1'); // one
echo superFunction($arr, 'list.second.val21'); // two2

Cпасибо!
Обновил.