$button = new GtkButton('Button');
$button->set_sensitive(false);

//Now i got this function, im not sure if its right...hehhe
function on_select(){
   //Some codes here for data display after selection
   //Output some array
   $data = array($id,$fname,$lname....);
   //I just check here if selected row is empty or not...

   if(!(empty($data[0]))){
       $button->set_sensitive(true);
       //But get some error like "Undefined function set_sensitive on a non-object on line 123
   }
}