public function getRandomKeys($number)
{
$number = (int) $number;
$count = $this->count();
if ($number === 0 || $number > $count) {
throw new \RangeException(sprintf('Number of requested keys (%s) must be equal or lower than number of elements in this array (%s)', $number, $count));
}
return array_rand($this->elements, $number);
}