<?php

    class Test
    {
        /**
         *
         */
        public function getName($num = 0)
        {
            $num = (int)$num;

            if ($num >= 0 && $num <= 99)  {
                return $this->get($num);
            } else {
                return $this->get((int)substr((string)$num, (strlen((string)$num) - 2), 2));
            }
        }

        /**
         *
         */
        private function get($num = 0)
        {
            if ($num >=11 && $num <= 19) {
                return 'Дней';
            } else {
                $num = (int)($num < 10 ? $num : substr((string)$num, 1, 1));

                switch (true) {
                    case $num == 1:
                        return 'День';
                    case $num >=2 && $num <=4:
                        return 'Дня';
                    default:
                        return 'Дней';
                }
            }
        }
    }