Will I get loot when defeating a boss in another player's world?
If a player summons me to his world to fight a boss, will I get loot from the boss after killing it or will only the host be rewarded with loot?
No, phantoms do not have the ability to gain any generic loot from their host's world. The only exception is items dropped by the host (and possibly other phantoms), which can be interacted with as normal.
The only thing phantoms earn from helping the host is souls (from all kills), humanity (from killing the boss), and possibly a Sunlight Medal (also from killing the boss).