如何使用include将一些php文件包含到另一个文件夹中

文件夹结构

/root
    /test1
        footer.php
    index.php

index.php

//works fine because test1 is on the same level as index.php
include('test1/footer.php');

文件夹结构

/root
    /test1
        footer.php
    /test2
        index.php

index.php

//does not work since in our current folder there is no folder "test1"
include('test1/footer.php');
//does work since we go back to root first
include('../test1/footer.php');