I was thinking, like you have java applets, it would be kewl to have php applets, witch would just support the same code as usual php but support events, but since PHP is not really object orientated like java, this is probably not so realistic..
But it would allow to make chat systems and email clients in PHP allot better since you wouldn't have to refresh the page to see wether there is a new message..
But yeah, probably like I said, not so realistic :o)