How to remove these horizontal lines in PHPStorm?
Generally speaking there are two kinds of such separators:
1. Actual method separators -- they appear before method/function body. I do not see them on your screenshot so you must have removed them already.
It's controlled by Settings/Preferences | Editor | General | Appearance | Show method separators option:
2. Other separators -- before class/namespace/etc. These are visible on your screenshot.
Thing is: you cannot "remove" them in the same way -- only mask them (make invisible). For that:
Settings/PreferencesEditor | Color Scheme | GeneralCode | Method separator color
Either remove any assigned colors (try this first) or assign the same color as background.
