登录时将NOW(）插入MySQL列

当用户登录到他们的帐户时，我试图将当前的DATETIME插入MySQL。屏幕上没有显示任何错误，进行了登录，但NOW（）没有插入。我检查了MySQL中的列声明，它是正确的——DATETIME

我的PHP低于

         if(isset($_POST['username']) && isset($_POST['password'] )) {         
          $query = mysql_query("SELECT username, password, image_name
                  FROM forum_login
                  WHERE username = '$username'
                  AND password = '$password'") or die (mysql_error());
                  $user = mysql_num_rows($query); 
                      if($user == 1) { 
                  $row = mysql_fetch_assoc($query);
                  $_SESSION['username'] = $row['username'] ;
                  $_SESSION['image_name'] = $row['image_name']  ;
                  $login_time = mysql_query("UPDATE forum_login
                                             SET last_login = 'NOW()'
            WHERE username = '".$_SESSION['username']."'") or die (mysql_error());
              header('Location: http://127.0.0.1/tapmichiana/'); 
              die;
                } 
             else 
             {        
                         echo "<div class='OpenError' >";
                         echo "Username & Password Combination Is Wrong"; 
                         echo "</div>";
                 }

我还没有显示数据净化和清理的部分脚本