Note that under Windows, a slash-rooted path will resolve on the local drive, and *not* necessarily C:\.
For example:
M:\>php -r "print realpath('/AUTOEXEC.BAT');"
[prints nothing, because there is no M:\AUTOEXEC.BAT]
But:
M:\>C:
C:\>php -r "print realpath('/AUTOEXEC.BAT');"
C:\AUTOEXEC.BAT
Same script, different response depending on current drive.
I'm inclined to argue that this function *should* use the value of %SystemDrive% as the "slash root" base.