php select disable submit no value

1$(document).ready(function() {
2     $("#names").change(function () {
3          var str = "";
4          if ($("#names option:selected").val()=='')
5            {
6
7                $('#jsbutton').attr('disabled', 'disabled');  
8            }
9            else
10            {
11
12                $('#jsbutton').removeAttr('disabled');
13            }
14        });
15});