PHP "file_exists" on Windows 7 (UserName)

i'm doing this check via PHP... i'd like to know if a program is installed

if (file_exists("C:\Users\USERNAME\AppData\Roaming\xXXx\xXXx\xXXx.exe"))

how do i get (retrieve automatically) the USERNAME?

let me say i'd like to do something corresponding to this (used in a batch file)... if exist %SystemDrive%\Users\%UserName%\AppData\Roaming\xXXx\xXXx\xXXx.exe

Thanks in advance