PHP页面的Apply Leave下拉菜单包含Annual Leave，No Pay Leave，Compensation Leave三项子菜单。要求点击子菜单时，查询数据库并在原网页内用表格输出查询的内容。生成PHP完整代码

以下是一个简单的PHP代码示例，实现了上述功能：

<?php
// 连接数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

$conn = mysqli_connect($servername, $username, $password, $dbname);

// 检查连接是否成功
if (!$conn) {
  die("Connection failed: " . mysqli_connect_error());
}

// 处理提交的表单数据
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $leave_type = $_POST["leave_type"];
  
  // 查询数据库
  $sql = "SELECT * FROM leaves WHERE type='$leave_type'";
  $result = mysqli_query($conn, $sql);

  // 输出查询结果
  if (mysqli_num_rows($result) > 0) {
    echo "<table>";
    echo "<tr><th>ID</th><th>Type</th><th>Start Date</th><th>End Date</th></tr>";
    while($row = mysqli_fetch_assoc($result)) {
      echo "<tr><td>" . $row["id"]. "</td><td>" . $row["type"]. "</td><td>" . $row["start_date"]. "</td><td>" . $row["end_date"]. "</td></tr>";
    }
    echo "</table>";
  } else {
    echo "No results found.";
  }
  
  // 关闭数据库连接
  mysqli_close($conn);
}
?>

<!-- HTML表单 -->
<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
  <label for="leave_type">Leave Type:</label>
  <select id="leave_type" name="leave_type">
    <option value="Annual Leave">Annual Leave</option>
    <option value="No Pay Leave">No Pay Leave</option>
    <option value="Compensation Leave">Compensation Leave</option>
  </select>
  <br><br>
  <input type="submit" value="Submit">
</form>

注意，以上示例代码仅演示了如何连接数据库、查询数据和输出结果，实际应用中需要根据具体情况进行修改和完善。另外，为了安全起见，需要对用户输入进行一定的验证和过滤，以防止 SQL 注入等攻击。