How to make a symbol like this? Due to the reputation limit, I can't upload an image. So I decribe the symbol by word, I hope you can understand it. My symbol is:
a phi with an underscore. phi is
\niphi.
Perhaps this does what you want. (Adapted from the Comprehensive Symbols Guide (available in your TeX Distribution.)
\documentclass{article}
\usepackage{upgreek,slashed}
\newcommand*{\ulphi}{{%
\declareslashed{}{-}{0}{-.62}{\upphi}\slashed{\upphi}}}
\begin{document}
\[
\ulphi
\]
\end{document}

Update
A similar question has been asked on the site with some more general solutions:
texdoc symbols should find it. If you look at package documentation from within your editor, then enter symbols as the package name.
– Alan Munn
Feb 01 '11 at 15:06
This link has probably been posted in response to various other "How do I get xxx symbol" questions, but anyway here it is again:
http://detexify.kirelabs.org/classify.html
Draw your symbol there, and the system will try to identify it (together with the LaTeX package and command that produces it).
is that what you mean or
?
– percusse Feb 04 '11 at 20:52