is safemode on?
if so, you cant. its a limitation of safemode.
mkdir creates the directory under a different owner than php. and if safemode is on, you cant write to a directory not owned by php.
you should be getting an error when you try if safemode is on though....
you could use php's ftp functions to log in via ftp and create the directories though, as safemode wont restrict that.