As i have discovered
stream_filter_append($cin, 'convert.quoted-printable-decode');
and
stream_filter_append($cin, 'convert.base64-decode');
don't decode stream line-by-line. These functions both first read the whole stream until EOF and then decode its data as string.
Is there any soution to decode base64 and quoted-printable encoded streams line-by-line?
Well, it was a mistake. These filters do convert streams sequentially, block by block.