Split text string into $first and $last name in php

Question

I'm developing a private message system that allows users to search for a user by their full name, e.g.: "George Washington".

I have two variables named $firstname and $lastname, and the search function orders results by relevancy (how many times you have messaged that person). How do I get a text field to split "George Washington" into $firstname="George" and $lastname="Washington"?