SETX doesn’t append path to system path variable

To piggy-back on @Endoro’s answer (I lack the rep to comment):

If you want to change the system-wide environment variables, you have to use /M, a la:

setx PATH "%PATH%;C:\Program Files\MySQL\MySQL Server 5.5\bin" /M

setx.exe is picky about placement of the /M, BTW. It needs to be at the end.