AS near as I can tell, this is simply a Windows problem, and you will need to to use uniqid("", true).
As Cygwin runs on the Windows platform:
PHP Manual wrote:Note:
Under Cygwin, the more_entropy must be set to TRUE for this function to work.
I think (but I am not an expert), that this is due to granularity differences between the POSIX/UNIX gettimeofday() and clock_gettime(), and the Windows GetSystemTime() and GetSystemAsFileTime(). POSIX is, at a minimum, 100 times more precise.