----- is used in smoke screens.

Smoke screen is a dense artificial smoke used in military areas to escape from enemies

$$\begin{gathered} {\mathrm{Ca}}_3{\mathrm{P}}_2+6{\mathrm{H}}_2\mathrm{O}→3\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+2{\mathrm{PH}}_3 \\ 2{\mathrm{PH}}_3+4{\mathrm{O}}_2→{\mathrm{P}}_2{\mathrm{O}}_5(\mathrm{white} \mathrm{dense} \mathrm{smoke})+3{\mathrm{H}}_2\mathrm{O} \end{gathered}$$

Thus PH₃ forms white dense smoke of P₂O₅

$∴$It is used in producing smoke screen.