```
var_dump(DIRECTORY_SEPARATOR) // string '\' (length=1)
var_dump(file_exists("C:/1212.txt")); // true
var_dump(file_exists("C:\1212.txt")); // false
var_dump(file_exists("C:".DIRECTORY_SEPARATOR."1212.txt")); // false

```

What does `DIRECTORY_SEPARATOR` do? Why is the above case false when using `DIRECTORY_SEPARATOR`?