\newenvironment{boxed}
    {\begin{center}
    \begin{tabular}{|p{0.9\textwidth}|}
    \hline\\
    }
    { 
    \\\\\hline
    \end{tabular} 
    \end{center}
    }
%--------------------------------------------------

Below this line a boxed environment is used

\begin{boxed}
This is the text formatted by the boxed environment
\end{boxed}

This text is again outside the environment