gistfile1.php

$a = array('foo' => array('bar' => array('baz' => 1)));

function resolve(array $a, $path, $default = null)
{
  $current = $a;
  $p = strtok($path, '.');

  while ($p !== false) {
    if (!isset($current[$p])) {
      return $default;
    }
    $current = $current[$p];
    $p = strtok('.');
  }

  return $current;
}

var_dump(resolve($a, 'foo.bar'));

// array('baz' => 1)