Any server that pushes something to its client needs a constant connection. Then its down to the protocol being used. If its just regular pop then the client will poll on a regular basis.
AFAIK, there isn't a 'push email' type of protocol.
How php 5.3 will support this is a vague question. If you're running a php pop daemon, then it would be down to your own code to support this. PHP doesn't generally act as a server in its own right (although it has the ability via your own code) so unless PHP is planning to offer this as a class or library I don't really think it's going to be a new feature.