Here's a mapping that might help.
:imap \p <esc>diwa$<C-R>" = $_POST['<C-R>"'];
<esc> - takes you out of insert mode
diw - deletes the word under the cursor into the default register
a - puts you back into insert mode
<C-R>" - inserts the contents of the default register
When the mapping is done, you're left in insert mode, so you can just hit <enter> and continue with the next line.
To use this, type just the symbol name on a line and immediately follow it with
You can put the mapping itself in your
~/.vim/after/ftplugin/php.vim on Unix, or
~\vimfiles\after\ftplugin\php.vim on Windows.