How may the following transformation be carried out (in not more than six steps)?
“Benzaldehyde to cyanobenzene”
\begin{matrix} \overset{K_2Cr_2O_7}{\underset{H^+}{\xrightarrow {\hspace{2 cm}} }} \\\\\\\\\\ \end{matrix} \begin{matrix} \overset{PCl_5}{\xrightarrow {\hspace{2 cm}} } \\\\\\\\\\ \end{matrix}
\begin{matrix} \overset{NH_3}{\xrightarrow {\hspace{2 cm}} } \\\\\\\\\\ \end{matrix} \begin{matrix} \overset{P_2O_5}{\underset{-H_2O}{\xrightarrow {\hspace{2 cm}} }} \\\\\\\\\\ \end{matrix}