I was having a empty outline problem after recently having installed Eclipse Oxygen, the latest version, and immediately imported a project by folder. The excitement was short lived. I even briefly considered installing an older version (neon), before I randomly found a solution to the missing outline contents.
Getting back your PHP file outline
To solve the problem, I simply right-clicked on my project in Explore and converted my project to a PHP project. This also added some extra stuff to my project, "PHP Language Library", as seen in the below screenshot. And most importantly, it fixed the problem with the missing outline.
Simply right click on the project in the Explore, then choose: Configure -> Convert to PHP project...