Refactoring code test PHP

test-1.php

<?php

/**
 * Is the bar open
 *
 * @return bool 
 */
function isBarOpen($day)
{
    if ($day) {
        if (is_string($day)) {
            if ($day == 'friday' || $day == 'Friday') {
                return true;
            } elseif ($day == 'saturday' || $day == 'Saturday') {
                return true;
            } elseif ($day == 'sunday' || $day == 'Sunday') {
                return true;
            } else {
                return 0;
            }
        } else {
            return 0;
        }
    } else {
        return 0;
    }
   
}

A junior developer has written a function which will determine if the bar is open or not. It should return true or false. Your task is to help the junior developer to improve the code. The test is limited, so you don't need to change anything outside line 10-27.

Backward compatibility is not required.