Right, the difference between "fashionable" and "unfashionable" is just what the cool kids on HN say it is.

To take your example on a stroll, PHP is like regular (non-skinny) blue jeans. Everyone knows them, most people use them, every store carries them, at one point in history they were revolutionary, they're probably never going to go away. But the cool kids say skin-tight crazy-colored jeans are the new hotness...