Why can't I use "/" in a php file to access files in the root directory of the website, but html can?

Question

1. When I wrote a demo, I found that there is no way to use "/" to introduce files in the root directory of the website, but it can be done in html. Please give me an answer.

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Answer 1

In php, /代表的是文件系统的根目录，而非网站根目录.
在html中, /represents the root directory of the website, not the root directory of the file system.