单选按钮中的数据未插入数据库

Question

我正在为我的系统做这个单选按钮（如星级）。我已经尝试了一切，但我的数据没有插入我的数据库。当我运行时并没有说我有错误但是当我提交我的按钮时它只显示“更新数据时出错”

这是我的单选按钮代码

 <legend>Star Rating</legend><br>

    <center>
    <form action="ratingprocess.php" method="post">

Contractor Name: <select name="username"><br>
<option selected>Select Contractor</option>
    <?php
    $con=mysqli_connect("localhost","root","","fyp");
    if (mysqli_connect_errno())
    {echo "Failed to connect to MySQL: " .     mysqli_connect_error();}
    $sql="SELECT * FROM contractor";
    $result = mysqli_query($con, $sql);
    while($row = mysqli_fetch_array($result))
  {    
    echo '<option value='.$row['username'].'>'.$row['username'].'</option>';
      }
?>
    </select><br>

<br><br>

    <div class="star">
        <input type="radio" name="star" class="star-1" id="star-1" value="1" />
        <label class="star-1" for="star-1">1</label>
        <input type="radio" name="star" class="star-2" id="star-2" value="2" />
        <label class="star-2" for="star-2">2</label>
        <input type="radio" name="star" class="star-3" id="star-3" value="3" />
        <label class="star-3" for="star-3">3</label>
        <input type="radio" name="star" class="star-4" id="star-4" value="4" />
        <label class="star-4" for="star-4">4</label>
        <input type="radio" name="star" class="star-5" id="star-5" value="5" />
        <label class="star-5" for="star-5">5</label>
    </div>

<br>
<input type = "submit" value = "Submit"> <t> </t>
</center>
</div>
</fieldset> 
</form>

这是我的PHP代码

<?php
    include("connection.php");
    $con = mysqli_connect("localhost","root","","fyp");
    $username=$_POST["username"];
    $rate=$_POST["star"];


   if (!$con)
    {
     die("Cannot execute sql.");
    }
    $update_sql="UPDATE contractor SET star='$rate' WHERE username = '$username'";
    $sql_result=mysqli_query($con,$update_sql);

    if (!$sql_result)
    {
    die ("Error in updating the data");
    }
      else
          echo "Succesfully send the rating.";

?>