在網上看到這篇文章，轉載來記錄下，估計會用到。
常見需求：把一個父欄目下的子欄目循環出來，每個子欄目下再顯示最新的幾篇內容。

<?php
$servername = "localhost";
$username = "root";
$password = "root";
$dbname = "test";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

//一级菜单id
$pid = $_REQUEST['pid'];
$sql = "SELECT  *  FROM `master_class` WHERE pid=" . $pid;

//分类数据
$class_result = $conn->query($sql);

//最终数据json
$data_json = array();
//文章数据
$tmp_art=array();
//分类
$tmp_cla=array();

//有结果 
if ($class_result->num_rows > 0) {
    // while 输出数据
    while ($row = $class_result->fetch_assoc()) {
        //得到classid
        $classid = $row['id']; 
        $article_sql = "SELECT  *  FROM  `master_article` WHERE `classid`=" . $classid; 
        $tmp_cla['id']=$row['id'];
        $tmp_cla['name']=$row['name'];
        $tmp_cla['pid']=$row['pid'];
        
        //查询文章数据
        $article_result = $conn->query($article_sql);
        if ($article_result->num_rows > 0) {
            // while 输出数据
            while ($r= $article_result->fetch_assoc()) { 
               $tmp=array('id'=>$r['id'],'name'=>$r['name'],'classid'=>$r['classid']);
               $tmp_art[]=$tmp;
            }
            $tmp_cla['article']=$tmp_art;
            $tmp_art=null;
        } 
        $data_json[]=$tmp_cla;
    }

} else {
    echo "0 结果";
}

echo json_encode($data_json);

$conn->close();
?>