php - PDO Update not working -

im trying pdo update mysql database. not showing errors, neither updating table.

here code im using. user selects post value drop down list.

 if (isset($_post['changelearningstyle'])){  if (isset($_post['learning'])){  $var6 = $_post['learning'];  $stmt8 = $db->prepare("update users set learningstyle = $var6 username = ?  ,     learningstyle = ?");  $stmt8->execute(array($id, $learningstyle));  $alert = '<div id="title1">your learning style have been updated</div>';  } } /**and html looks  this: **/ <form name="changestyle" method="post"> <div id="resetlearningstyle"> <div id="tab6">  <h4 id="black">learning style</h4>  <h5>current style: <?php echo($learningstyle) ?></h5>  <select id="learning" name="learning" class="span3">   <option value="">select style</option>   <option value="auditory">auditory</option>   <option value="visual">visual</option>   <option value="persuasive">persuasive</option>   <option value="active">active</option>       </select>   </div>   <input class="btn btn-inverse" type="submit" name="changelearningstyle" value="reset    learning style">   </form>  /**table structure **/ 

-- table structure table users

  create table `users` (  `id` int(20) not null auto_increment,  `fname` varchar(200) not null,  `lname` varchar(200) not null,  `username` varchar(200) not null,  `password` varchar(200) not null,  `country` varchar(200) not null,  `rootlanugage` varchar(200) not null,  `learningstyle` varchar(200) not null,  `language` varchar(200) not null,  `icon` varchar(200) not null,   primary key  (`id`) ) engine=myisam  default charset=utf8 auto_increment=31 ; 

thank in advance!

to make show errors, add line of code right after connecting db:

$db->setattribute(pdo::attr_errmode, pdo::errmode_exception);