problem with unicode strings in php

hi
if you saw, we have a function with name preg_split in php, and if we using it like below code :
preg_split('//', $str, -1, PREG_SPLIT_NO_EMPTY )
it will get us array of characters in $str, but you know this function is not for unicode strings.
for unicode, i could find one function with name mb_split. but this function cannot separate characters of unicode string.
if you have any suggestion, please tell me and help me.
how can i separate characters of unicode strings in php.