The Oxford Advanced American Dictionary lists the word “Philippine” as a pertaining adjective that means “from or connected with the Philippines.” None of the other leading dictionaries that I consulted explains why the letter “s” is dropped from the place-name “Philippines” to form the pertaining adjective, but I would think it’s for the sake of euphony and ease of expression or articulation. Saying “the Philippines capital of Manila” does sound awkward as compared to saying it as “the Philippine capital of Manila,” which comes much easier to the tongue and looks simpler grammatically.
