Fontiforcation for Deprecation

@zonuexe

The file `php7.el` defines some constants for deprecated functions. How do you feel about us adding some code to `php-mode` that would highlight those functions in a slightly different way; I'm not sure how to display them, but something close to what errors look like with our syntax highlighting code. Showing deprecated functions should probably also be toggable.

I have not done anything with PHP 7 so unfortunately I cannot provide a pull request at the moment. I will try to look into it and present some code if you feel it is a good idea.

`--ejmr`