PH is "posthumous," awarded after his death. John 15:13 is a chapter and verse of the Book of John in the bible, which reads, "Greater love has no one than this: to lay down one’s life for one’s friends."
Various veterans' organizations place flags at military gravesites on Memorial Day or Fourth of July.
Edit: Although Mrsjvb has answered correctly about the quarters indicating a visit, it's interesting to know the practice started with stones indicating a visit to the grave at a Jewish cemetery. (The things you research, huh?) This was to mark where graves were because rabbis became impure if they approached too closely without the ritual prayers (a practice since abandoned) and many a grave was just a burial with rocks on top, no headstone or anything.If there were small stones, that indicated it was a burial site. Many cemeteries in the US are green rather than stony dry earth, so people substitute other small objects.