Plus and Minus Icons/Buttons Not Visible on PHP Pages in Visual Studio

Question

Whenever I've used Visual Studio I've taken the little plus's and minus's on the left side of the page for granted. These little symbols hide blocks of code when in a minus state and expand in a plus state. I don't know what these buttons are called. They basically allow a developer to view or hide functions within a page. It is really useful and not having it is difficult because you cannot identify opening and closing brackets as easily.

How do I enable this for PHP pages? It works fine for .aspx and .cshtml pages but .php always misses it.

UPDATE: I have partially fixed the problem by going to edit->outlining but it only works for the first level. By "first level" I am referring to the first level of stuff on the page. For instance, if I have a function declared it has the little plus and minus to hide content but if there is an if statement in the function the if statement has no outlining feature. This makes it really hard to edit nested if statements.