The P.T. playable demo that Sony announced at Gamescom and released earlier today on the PlayStation Store was actually a teaser for another game: a new Silent Hill from Metal Gear creator Hideo Kojima and director Guillermo del Toro.
The PlayStation 4 "interactive teaser" was apparently a stealthy, creative way to announce the new Silent Hill game that Kojima has been teasing for months. At the end of the P.T. demo, a character is shown walking through a quiet, foggy city street. The names Hideo Kojima and Guillermo del Toro appear on screen just before the main character turns around, revealing that he is played by The Walking Dead's Norman Reedus.
The game is apparently called Silent Hills, according to a title card backed by Akira Yamaoka's Silent Hill theme.
You can see a replay of the end of the P.T. demo and the Silent Hills reveal in this Twitch video (skip to 1:16:00) by SoapyWarpig.
Sony announced P.T. as a playable interactive teaser for PlayStation 4 earlier today. The game was credited to 7780s Studio, a heretofore unknown game developer. Kojima employed a similar ruse when announcing The Phantom Pain by the fictitious Moby Dick Studios, a game later revealed to be Metal Gear Solid 5.
Update: Soapy Warpig has uploaded the key moment of the Silent Hills reveal to her YouTube channel, complete with excited reaction.