How to get the Peso sign (₱) in LaTeX?

If one needs the ability to adjust the vertical space between the cross-lines, then this can work:

\documentclass[10pt]{article}
\usepackage{stackengine}
\newcommand\pesos{%
  \stackengine{-1.38ex}{P}{\stackengine{-1.2ex}{$-$}{$-$}{O}{c}{F}{F}{S}}{O}{c}{F}{T}{S}}
\begin{document}
35\pesos ~\sffamily 35\pesos ~\ttfamily 35\pesos
\end{document}

If using a \scriptstyle= sign is sufficient, a nested stack is avoided, at the expense of a \scalebox:

\documentclass[10pt]{article}
\usepackage{stackengine,graphicx}
\newcommand\pesos{\stackengine{-.85ex}{P}{\scalebox{1.15}[1]{$\scriptstyle=$}}{O}{c}{F}{T}{S}}
\begin{document}
35\pesos ~\sffamily 35\pesos ~\ttfamily 35\pesos
\end{document}

This assumes that the arc of the P is half the height of the letter.

\documentclass{article}

\makeatletter
\DeclareRobustCommand{\PHP}{%
  \begingroup
  \leavevmode\,\vphantom{P}%
  \dimen\z@=.5\fontcharht\font`P\relax
  \dimen\tw@=0.33333\dimen\z@
  \ooalign{%
    \raisebox{\dimexpr\dimen\z@+2\dimen\[email protected]}{\rule{\fontcharwd\font`P}{0.4pt}}\cr
    \raisebox{\dimexpr\dimen\z@+\dimen\[email protected]}{\rule{\fontcharwd\font`P}{0.4pt}}\cr
    P\cr
  }%
  \,\endgroup
}
\makeatother

\begin{document}

\PHP\quad\textbf{\PHP}\quad{\Large\PHP}

100\PHP

\end{document}