Is it possible to get an inventory of all physical hard disks on a linux server?

In the case of hardware raid you would have to use a controller utility as provided by the manufactured of the raid controller.

Unfortunately there is no generic way to do it that would work with every HW raid vendor out there.

The RAID controller vendor usually provides an interface to the disks behind it. 3ware has the 3DM web management and also a CLI utility. Dell also provides this via OpenManage Server Administrator. Not sure about other vendors, but I would imagine the same.

Hardware RAID works by combining disks together and presenting them as devices on the hardware level. There's no generic way for a linux machine to detect that and probe for the real disks behind the hardware raid. However, most hardware raid devices come with some sort of linux utility program that provides info about the raid device. If you find that utility for your particular raid device, you can use it to discover the actual disks attached to the raid.