$\displaystyle \widehat{phi}_n$
And I expect the OP wanted the symbol $\displaystyle \displaystyle \begin{align*} \phi \end{align*} $ or $\displaystyle \displaystyle \begin{align*} \varphi \end{align*} $ or $\displaystyle \displaystyle \begin{align*} \Phi \end{align*} $.
Wiggle the cursor over the symbols for the code
Thanks so much to Plato and Prove It for the help. I was copying code from a post and couldn't get it to work using [tex] tags. When I saw you used [TEX] I tried it and it worked.
Now to reply to hacking post to put it back on top. Im also having problems.