$('.button1').click(function() {
window.location = "www.example.com/index.php?id=" + this.id;
});
First of all using window.location is better as according to specification document.location value was read-only and might cause you headaches in older/different browsers. Check notes @MDC DOM document.location page
And for the second - using attr jQuery method to get id is a bad practice - you should use direct native DOM accessor this.id as the value assigned to this is normal DOM element.