java将json发送到php并插入到mysql中，但返回null

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;

import java.io.IOException;

public class Main {
    public static void main(String[] args) throws IOException {
        // 构造JSON数据
        String json = "{\"name\":\"John\",\"age\":30}";

        // 发送POST请求
        HttpClient httpClient = HttpClientBuilder.create().build();
        HttpPost request = new HttpPost("http://your-php-server.com/insert.php");
        request.setEntity(new StringEntity(json));
        HttpResponse response = httpClient.execute(request);

        // 处理响应
        String result = EntityUtils.toString(response.getEntity());
        System.out.println(result);
    }
}

<?php
// 接收并处理POST请求中的JSON数据
$data = json_decode(file_get_contents('php://input'), true);

// 连接到MySQL数据库
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database";

$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接是否成功
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 将数据插入到MySQL中
$sql = "INSERT INTO your_table (name, age) VALUES ('".$data["name"]."', ".$data["age"].")";
if ($conn->query($sql) === true) {
    echo "数据插入成功";
} else {
    echo "数据插入失败: " . $conn->error;
}

// 关闭连接
$conn->close();
?>