Peer-to-Peer (P2P)
Peer-to-peer, or P2P, is a network architecture where participants, called peers, communicate directly with one another instead of routing every exchange through a central server. Each peer can act as both a client and a server, in contrast to the traditional client-server model.
P2P is not a common architecture for PHP applications, since PHP scripts are typically executed on demand behind a web server and do not keep a long-running process able to accept inbound peer connections. It is more often encountered as a concept PHP code interacts with indirectly, for instance a script that talks to a BitTorrent tracker, a blockchain node, or a distributed hash table over sockets, or a long-running CLI daemon built with something like ReactPHP or Swoole.