How to rewrite this command using \newenvironment?

Question

I once defined a customized latex command, which works well. And now I would like to convert it into a latex environment using \newenvironment command. However, I failed after trying several ways. Could you help me, please?

The definition of my own latex command is

\newcommand{\smallbox}[1]{
    \tikzstyle{mybox} = [draw=red, rectangle,inner sep=5pt, inner ysep=5pt]
    \begin{center}
        \begin{tikzpicture}
            \node [mybox] (box){
                \begin{minipage}{0.85\textwidth}
                     #1
                \end{minipage}
            };
       \end{tikzpicture}
    \end{center}
}

Let's call the to-be-defined enviroment "smallenvironment" for the moment.

And I would like that, the following command

\smallbox{This is a small box!}

is equivalent to

\begin{smallenvironment}
    This is a small box!
\end{smallenvironment}

Thanks!

Answer 1

With the help of \NewEnviron from the environ package

\documentclass{article}

\usepackage{tikz}
\usepackage{environ}

\NewEnviron{smallenvironment}{%
    \tikzstyle{mybox} = [draw=red, rectangle,inner sep=5pt, inner ysep=5pt]%
    \begin{center}%
        \begin{tikzpicture}%
            \node [mybox] (box){%
                \begin{minipage}{0.85\textwidth}%
                     \BODY
                \end{minipage}%
            };%
       \end{tikzpicture}%
    \end{center}%
}

\begin{document}

\begin{smallenvironment}
This is a small box!
\end{smallenvironment}

\end{document} 

Another way, as egreg notes in his comment, is to keep your definition of \smallbox and simply say

\NewEnviron{smallenvironment}{\smallbox{\BODY}}

---

EDIT

Note that an environment like the above can be obtained in a much simpler way with the help of the mdframed package.

Take a look at the following MWE

\documentclass{article}

\usepackage{tikz}
\usepackage{environ}
\usepackage{mdframed}

\newcommand{\smallbox}[1]{
    \tikzstyle{mybox} = [draw=red, rectangle,inner sep=5pt, inner ysep=5pt]
    \begin{center}
        \begin{tikzpicture}
            \node [mybox] (box){
                \begin{minipage}{0.85\textwidth}
                     #1
                \end{minipage}
            };
       \end{tikzpicture}
    \end{center}
}

\NewEnviron{smallenvironment}{\smallbox{\BODY}}

\newmdenv[%
    linecolor=red,
    innerleftmargin=5pt,
    innerrightmargin=5pt,
    leftmargin=20pt,
    rightmargin=20pt
  ]{smallmdframe}

\begin{document}

\begin{smallenvironment}
This is a small box!
\end{smallenvironment}

\begin{smallmdframe}
This is a small box!
\end{smallmdframe}

\end{document} 

and at its output

Answer 2

Karl has already shown you about forming a newenvironment using environ. I will point out some improvements to your tikzpicture code.

1. Use tikzset instead of tikzstyle.
2. No need of specifying ysep when you have already inner sep.
3. No need of using minipage inside the node. You can specify text width= 0.85\textwidth, minimum width = 0.85\textwidth in node options, making it more tikz way.
4. Instead of begin{center} and \end{center} you may use \centering if you want to avoid extra vertical space.

Code:

\documentclass{article}

\usepackage{tikz}
\usepackage{environ}

\NewEnviron{smallenvironment}{%
    \tikzset{mybox/.style = {draw=red, rectangle,inner sep=5pt}}%
    \begin{center}%     %% \centering         
        \begin{tikzpicture}%
            \node [mybox,text width= 0.85\textwidth, minimum width = 0.85\textwidth] (box){%
               \BODY
            };%
       \end{tikzpicture}%
       \end{center}    %% \par %% if you use \centering
}

\begin{document}
Some text

\begin{smallenvironment}
This is a small box!
\end{smallenvironment}

Some text
\end{document}

With \centering, you get