\documentclass{report}
\usepackage{amsmath}
\makeatletter
\newcommand*{\spl@}[1]{%
  \[\begin{split}#1\end{split}\]%
}
\newenvironment{spl}{%
  \collect@body\spl@
}{}
\makeatother
\begin{document}
\[ \begin{split}
a&=b \\
 &=c \\
 &=d
\end{split} \]
\begin{spl}% the new defined environment
z&=y \\
 &=x \\
 &=w
\end{spl}
\end{document}
