A few questions for beginners

A few questions for beginners! First, let me explain the situation. The basic functions of PHP can be to display files in the directory. you can upload and download files. you can click here to view the files and return to the previous directory. 1. the header download function can only download files from the root directory. files in subdirectories under the root directory cannot be downloaded. how can I modify the function to download objects in the subdirectories? 2. how can I create a button for beginners!
First, let me explain the situation. The basic functions of PHP can be to display files in the directory. you can upload and download files. you can click here to view the files and return to the previous directory.
1. the header download function can only download files from the root directory. files in subdirectories under the root directory cannot be downloaded. how can I modify the function to download objects in the subdirectories?
2. how to create a button or link so that the button is displayed as the previous page after the button is clicked.
3. how can I restrict the viewing of only the contents in the root directory? I cannot return the upper-level contents in the root directory.
I hope the experts can give you some code to explain that I still won't write my answers!

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I can't solve it. I want to reply to this post. I hope someone else can give me some advice and I want to learn it. after reading it for a few days, I really can't understand it ......
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2: This is paging...
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Are you going to ask someone to write one for you?

You post the code you need to modify so that everyone can raise their comments.
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Paste the code
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If (! Empty ($ _ GET [fn]) {
// ----------------------------------------- Download --------------------------------------------------

$ Fn = $ _ GET ['FN '];
$ Basename = pathinfo ($ fn, PATHINFO_BASENAME );
Ob_end_clean ();
Header ('content-Encoding: none ');
Header ('content-Type: text/plain ');
Header ('content-Length: '. filesize ($ fn ));
Header ('content-Disposition: attachment; filename = "'. $ basename .'"');
Readfile ($ fn );
Exit ();

}