Елизавета Цезурк

function group_days($block) {
	$week = array('Пн', 'Вт', 'Ср', 'Чт', 'Пт', 'Сб');
	$res = [];

	foreach ($block as $id=>$day) {
		if ($id == 0) {
			$period = [$day];
		} else {
			if (array_search($day, $week)-array_search(end($period), $week)==1) {
				$period[1] = $day;
			} else {
				$res[] = $period;
				$period = [$day];
			}
		}
	}

	$res[] = $period;
	
	$res = array_map(
		function($el) {return implode('-', $el);},
		$res
	);

	return implode(', ', $res);
}