I'm experimenting with my own commands and environments and now I'm facing those problems:

1. How to create command \foo{parameter}[optional] or environment called \begin{bar}{parameter}[optional]?
2. How to create command \foo[optional_1]...[optional_n]{parameter}

I've tried

\newcommand{\foo}[3][][]{#1#2#3}    - failed
\newcommand{\foo}[3][2][][]{#1#2#3} - failed

Does anyone know some hint? Thanks a lot.