Well, I don't know how to do it in one query, but I know how to do it all in a single operation.
SET @u = UUID(); UPDATE t SET f = @u WHERE id = 5;
As you know, I don't use PHP. That code will work from the MySQL command line or from a MySQL query issued via JSP or ASP or ASP.NET code. But I understand that PHP does not allow multiple MySQL queries per call to mysql_query( )
so I don't know how you would do it with PHP.