从下拉列表中插入值时没有错误，但不会插入

我有一个下拉循环，当我尝试从下拉列表中选择一个数字时，它会回显出来。 但是当我插入它时，它不会执行或不会显示在数据库中。 但是当我将 $_POST 值更改为静态数字时，它会插入并显示。如 $x=$_POST['类别'];它不插入，但$x=3;它插入到数据库中这是我得到的：

/*The Drop Down*/
 <select name="category">
 <?php 
 $query=mysql_query("SELECT * FROM deliverable_category");
 while($row=mysql_fetch_assoc($query)){ ?>
 <option value="
 <?php echo $row['deliverable_category_id'] ; ?> ">
 <?php echo $row['deliverable']  ;  ?> 
 </option>
 <?php } ?>
 </select>
/*The Query*/
  $name=$_POST['title'];
  $deliverable_desc=$_POST['deliverable_desc'];
  $category_id=$_POST['category'];
  $adviser=$_SESSION['user_id'];
         $con=mysqli_connect("localhost","root","","obeo") ;
                     if(mysqli_connect_errno())
                     {
                         echo "<br/>Error connecting to database!";
                         exit();
                     }else
                     {
                       echo "<br/> ";
         $sqlstmt="insert into deliverable(deliverable_title,deliverable_desc,adviser_id,deliverable_category_id) values(?,?,?,?)";


            $stmt=$con->prepare ($sqlstmt);
            $stmt->bind_param('ssss', $name,$deliverable_desc,$adviser,$category_id);
            $stmt->execute();