According to a user who received a physical copy of the PC version of the game early, The Phantom Pain does not seem to have the game on the disc. Instead, the only file on the disc appears to be an installer for Steam, in which users can proceed to download their copy of the game from there.
https://twitter.com/graphure/status/637981198229368836
It is rather odd that The Phantom Pain comes with a disc at all, seeing as how it is just a Steam installer on it. A simple voucher code would do, but this seems to defeat the purpose of releasing it on a disc at all. Metal Gear Solid V: The Phantom Pain will be released on September 1, 2015.
Source: VG24/7