include require在php中的区别
1、include在引入不存文件时产生一个警告且脚本还会继续执行，
require则会导致一个致命性错误且脚本停止执行。
2、include()是有条件包含函数，而 require()则是无条件包含函数。
3、include有返回值，而require没有
4、include()执行时需要引用的文件每次都要进行读取和评估,
require()执行时需要引用的文件只处理一次(实际上执行时需要引用的文件内容替换了require()语句)
可以看出若有包含这些指令之一的代码和可能执行多次的代码,则使用require()效率比较高,
若每次执行代码时相读取不同的文件或者有通过一组文件叠代的循环,就使用include(),
require通常使用方法，这个函数通常放在 PHP 程序的最前面，PHP 程序在执行前，就会先读入 require 所指定引入的文件，使它变成 PHP 程序网页的一部份。常用的函数，亦可以这个方法将它引入网页中。
include通常使用方法，这个函数一般是放在流程控制的处理部分中。PHP程序网页在读到 include 的文件时，才将它读进来。这种方式，可以把程序执行时的流程简单化
incluce在用到时加载
require在一开始就加载
_once后缀表示已加载的不加载
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