Change variables from b: to g:

[lukevers]

Not sure if you'd want this, so feel free to close this PR if you'd rather keep them on the buffer. I don't mind keeping my fork open for me to just use.

I had a problem where when I opened a new file from NERDTree, php-foldexpr didn't know anything about my previous buffer settings.

Love the plugin by the way-nice job!