此错误是什么意思使用未定义的常量selectedClientName-假定为“ selectedClientName”

Question

当尝试检索从下拉列表中选择的数据时，收到此错误“使用未定义的常量selectedClientName-假定为'selectedClientName'”。

它说错误在这行代码的第47行上

if (isset($_POST[selectedClientName])) { 

在下面的代码中。

<?php
   $result=mysqli_query($conn, "SELECT clientName FROM clients");
   echo "<left>";
   echo "<h1>Hotel Data Collection</h1>";
   echo "<hr/>";
   echo "<p> Choose Client: </p>";
   echo "<select name='selectedClientName'>";
   echo "<option>--Clients--</option>";
 while ($row=mysqli_fetch_array($result)) {
 $cn = $row['clientName'];
 echo "<option>$cn</option>";
}
 echo "</select>";
 echo "</center>";
?>
 <button type="submit" name="client-submit">Submit</button>
    </form>
  </section>
</div>

<?php
  if (isset($_POST[selectedClientName])) {
    $cn = $_POST['cn'];
     $query = "SELECT * FROM savedclientdata WHERE array_column         ['clientName'] = $cn";
$result = mysqli_query($conn, $query) or die('error getting data');
  }
?>

我的最终目标是让用户选择一个客户名称，然后显示与该客户相对应的数据。