Preheat oven to 350 degrees. Pour the melted butter evenly over the bottom of a 9-inch square baking pan. Spread the graham cracker crumbs evenly over the butter. Layer with the coconut, chocolate chips, peanut butter chips, one on top of the other, in the order listed. Drizzle the sweetened condensed milk over the top layer, and sprinkle with the walnuts. Bake for 30 minutes, or until lightly browned. Cool, then cut into bars.