\documentclass{article}
\begin{document}

\begin{equation}\label{eq1}
\sum_{i=0}^{i=10} \phi_i(3)
\end{equation}

\begin{equation}\label{eq2}
\int_{0}^{10} \phi_i(x)dx = 3
\end{equation}

Referring to [equation (\ref{eq1})] ...
\end{document}