iconv doesn't work in smarty application issue.

Hi!
 I just begin learning smarty,when I rewrite an application with
smarty,  I encounter a strange problem .the following simple
statement,seems doesn't work:

 $somestr = iconv("UTF-8","GB2312",$somestr);

when I move to another application directory,It works.  so I think it
just doesn't work in an smarty application directory.
but why?how can I use iconv in smarty?
besides,mb_convert_encoding doesn't work either.

any help is highly appricated!