Something managed to be clarified. Perhaps, it will be interesting.
If you remember, I have two servers, will call them W and T. T normally opens letters in these codings, W shows something like the Arab characters.
As iconv(php) is an interface to the linux utility iconv, it doesn't work at the server W where chroot is used. On the T server iconv transforms both of these codings normally.
codings. Mbstring, apparently, doesn't understand GB2312 (GBK), but understands ISO-2022-KR. Nevertheless, mbstring also doesn't work at W server. I think, the reason is also somehow connected to chroot.
I suppose, RC has problems with different codings by operation under chroot. Though I still hope to set up operation of iconv and mbstring.