在删除之前似乎无法弄清楚缺少什么

我正在尝试使用我的删除按钮，该按钮连接到一个单独的文件，该文件删除了数据库上表中的数据。

我已经足够成功，可以在显示数据时在数据中的每一行旁边放置一个按钮，但是当我收到以下错误消息时，我无法让它删除该行：

DELETE FROM users WHERE id = 
SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1

我相信这是一件很小的事情？但我就是想不通是什么。

管理员.php

<?php
$servername = "localhost";
$username = "root";
$pass = "root";
$dbname = "tutor_database";
// Create connection
$conn = new mysqli($servername, $username, $pass, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT id, FirstName, LastName, Role, Email, Username FROM users";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
    echo "<table><tr><th>ID</th><th>First Name</th><th>Last Name</th><th>Role</th><th>Email</th><th>Username</th><th>Delete</th></tr>";
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo "<tr><td>".$row["id"]."</td><td>".$row["FirstName"]."</td><td>".$row["LastName"].
        "</td><td>".$row["Role"]."</td><td>".$row["Email"]."</td><td>".$row["Username"]."</td></tr>";
?>
        <td>
       <form method="get" action="delete.php">
           <input type="hidden" name="rowid" value="<?php echo $row['id']; ?>">
           <input class="btn-default" type="submit" name="delete" value="Delete">
       </form>
     </td>
<?php
}
    echo "</table>";
} else {
    echo "0 results";
}
$conn->close();
?>

删除.php

<?php
$servername = 'localhost';
$username = 'root';
$pass = 'root';
$database = 'tutor_database';
try {
  $conn = new PDO("mysql:host=$servername;dbname=$database", $username, $pass);
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
//obtain the rowid from $_GET
if(isset($_GET['id'])){
  $id = ($_GET['rowid']);
}
  //sql to delete record.
  $sql = "DELETE FROM users WHERE id = ". $id;
  $conn->exec($sql);
  echo "Record deleted!";
  require 'adminPage.php';
} catch(PDOException $e) {
  echo $sql . "<br>" . $e->getMessage();
}
$conn = null;

?>

希望对此有一双新的眼睛。

谢谢。