Argument in the end of an environment trick is not working

Question

I was trying to define a new environment in which I needed to have an argument in the end code. Thanks to this post, I got that it is not possible without a trick. That is why I defined a command in the beginning containing the argument to use it in the end, but in vain... Can someone explain me why and/or fix this?

\newenvironment{newappendix}[2]%
{\def\headername{#1}%
\phantomsection%
\refstepcounter{section}%
\addcontentsline{toc}{section}{Appendix \thesection~- #1}%
\label{app:#2}}
{\fancyhead[R]{Appendix \thesection~- \headername}}

Without the command \headername on the last line it works fine but I need this argument here so... What to do?

EDIT Here is a MWE (by uncommenting the last line of the end of the definition of the environment you should get an error...

\documentclass{report}

\usepackage[demo]{graphicx}
\usepackage{hyperref}
\usepackage{fancyhdr}
\pagestyle{fancy}

\fancyhead[L]{{\bfseries Appendices}}%
\setcounter{section}{0}%
\renewcommand{\thesection}{\Alph{section}}%

\newenvironment{newappendix}[2]%
{\def\headername{#1}%
\phantomsection%
\refstepcounter{section}%
\addcontentsline{toc}{section}{Appendix \thesection~- #1}%
\label{app:#2}}
{\fancyhead[R]{Appendix \thesection~-}}% \headername}}

\begin{document}

\begin{newappendix}{first}{1}
\includegraphics[width=12cm,height=15cm]{test}
\end{newappendix}

\end{document}

Answer 1

You should use the mark feature of LaTeX and not an environment. Here's how I would do it.

\documentclass{report}

\usepackage[demo]{graphicx}
\usepackage{fancyhdr}
\pagestyle{fancy}

\usepackage{hyperref}

\fancyhead{}
\fancyhead[L]{\bfseries Appendices}
\fancyhead[R]{Appendix \rightmark}
\renewcommand{\thesection}{\Alph{section}}

\newcommand{\newappendix}[1]{%
  \clearpage % or the mark could be wrong
  \refstepcounter{section}%
  \addcontentsline{toc}{section}{Appendix \thesection~- #1}%
  \markright{\thesection \ -- #1}%
}
\begin{document}

\newappendix{first}\label{app:1}
\noindent\includegraphics[width=\textwidth,height=15cm]{test}

\newappendix{second}\label{app:2}
\noindent\includegraphics[width=\textwidth,height=15cm]{test}

\newappendix{third}\label{app:3}
\noindent\includegraphics[width=\textwidth,height=15cm]{test}

\newappendix{fourth}\label{app:4}
\noindent\includegraphics[width=\textwidth,height=15cm]{test}


\end{document}