Currently I'm using this solution:
$firstDay = new \DateTime('first day of this month');
$lastDay = new \DateTime('last day of this month');
The only issue I came upon is that strange time is being set. I needed correct range for our search interface and I ended up with this:
$firstDay = new \DateTime('first day of this month 00:00:00');
$lastDay = new \DateTime('first day of next month 00:00:00');