php session中文乱码

<?php
header('Content-Type: text/html; charset=utf-8');
?>

<?php
session_start();
ini_set('session.gc_maxlifetime', 3600);
ini_set('session.cookie_lifetime', 3600);
ini_set('session.use_cookies', 1);
ini_set('session.use_only_cookies', 1);
ini_set('session.name', 'PHPSESSID');
ini_set('session.serialize_handler', 'php');
ini_set('session.save_handler', 'files');
ini_set('session.save_path', '/tmp');
ini_set('session.cookie_httponly', 1);
ini_set('session.cookie_secure', 1);
ini_set('session.use_trans_sid', 0);

// 设置 Session 的字符编码为 UTF-8
header('Content-Type: text/html; charset=utf-8');
session_set_cookie_params(0, '/', '', true, true);
session_start();

CREATE DATABASE mydb CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
CREATE TABLE sessions (
    session_id char(32) NOT NULL PRIMARY KEY,
    session_data text NOT NULL,
    session_time int(11) unsigned NOT NULL DEFAULT 0,
    session_lifetime int(11) unsigned NOT NULL DEFAULT 0
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

<?php
header('Content-Type: text/html; charset=utf-8');

session_start();

// 设置 Session 数据
$_SESSION['username'] = '张三';

// 读取 Session 数据
echo $_SESSION['username'];
?>