days.php

<?php

final class Days
{
    const MONDAY = 1;
    const TUESDAY = 2;
    const WEDNESDAY = 3;
    const THURSDAY = 4;
    const FRIDAY = 5;
    const SATURDAY = 6;
    const SUNDAY = 7;

    const DAY_OF_WEEK_MAP = [
        self::MONDAY => 0b0000001,
        self::TUESDAY => 0b0000010,
        self::WEDNESDAY => 0b0000100,
        self::THURSDAY => 0b0001000,
        self::FRIDAY => 0b0010000,
        self::SATURDAY => 0b0100000,
        self::SUNDAY => 0b1000000,
    ];

    /**
     * @param array $days
     *
     * @return int
     */
    public static function toByte(array $days)
    {
        $accumulator = 0;

        foreach ($days as $day) {
            $byte = self::DAY_OF_WEEK_MAP[$day];

            $accumulator |= $byte;
        }

        return $accumulator;
    }

    /**
     * @param int $byte
     *
     * @return array
     */
    public static function fromByte($byte)
    {
        $days = [];

        foreach (self::DAY_OF_WEEK_MAP as $day => $part) {
            if ($part & $byte) {
                $days[] = $day;
            }
        }

        return $days;
    }

    /**
     * @param int $day
     *
     * @return string
     */
    public static function getDayName($day)
    {
        return strftime('%A', strtotime(sprintf('Sunday +%d days', $day)));
    }
}


$byte = Days::toByte([Days::MONDAY, Days::SATURDAY]);

foreach (Days::fromByte($byte) as $day) {
    echo Days::getDayName($day).PHP_EOL;
}