变量内的变量 - PHP

I'm trying to check if a folder is empty or not.

I use an Apache server and a Windows 10 operating system.

What's the problem with my code ?

function is_dir_empty($dir) {
  if (!is_readable($dir)) return NULL; 
  return (count(scandir($dir)) == 2);
}

$dir = "C:/Users/name/.spyder/Python2/data/$day-$month-$year/emptyFolder";
print "<br>$dir<br>";
if (is_dir_empty($dir)) {
  echo "the folder is empty"; 
}else{
  echo "the folder is NOT empty";
}