php更改包含文件的变量值

Question

我正在学习php，如何更改包含文件的变量值？

例如：

a.php只会

$name = 'john'

b.php

require_once 'a.php'
$name = 'james' // here i am changing value of $name

c.php

echo $name //here I want to show as 'james' not 'john' , how?