包含文件include，require，include_once，require_once

<?php
$a="你好，时间"

<?php
include('demo.php');
include('demo.php');//请求多次会输出多次
include('demo.php');
include('demo.php');

<?php
include('demo.php');//如果加载文件不存在提示waring级别错误，程序继续执行

<?php
require('demo.php');

<?php
require('demo.php');//文件找不到，程序致命错误

<?php
include_once('demo.php');
include_once('demo.php');//请求多次,检查之前导入过则忽略
include_once('demo.php');
include_once('demo.php');

require_once('demo.php');
require_once('demo.php');//请求多次,检查之前导入过则忽略