Can we assume by "USB" you are referring to "universal serial bus"? If so, are you talking about writing a local command line or PHP-GTK desktop application, or are you talking about a web application that somehow wants to access the client's USB port?
If a local app, it may be possible but I am not aware of any built-in functions nor any PEAR or PECL packages. You would probably need to locate a 3rd party PHP extension, most likely specific to the platform you want to run it on.
If a web app, it should not be possible as it would be a very nasty security risk.
