PHP multi-layer require

PHP multi-layer require problem, please php expert advice! I am doing. net, not too familiar with php. recently I want to build a private website and use php. I encountered a problem. suppose my website has such a directory structure/classes/. class. php/classes/B. class. php/c. phpa. class PHP multi-layer require problem, please php expert advice!
I am a. net Developer and I am not very familiar with php. recently I want to build a private website and use php. I encountered some problems.

Assume that my website has such a directory structure.

/Classes/a. class. php
/Classes/B. class. php
/C. php
A. class. php, B. class. php are two classes, and c. php is a page.

If. class. reference in php (require_once) B. class. php, c. php references. class. php, run c. php reports an error. B cannot be found. class. php

How can we solve such a multi-layer reference problem?

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C. How does php require?
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Is there no correct path?
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/Classes/a. class. php
$ Basedir = dirname (_ FILE __);
Require_once ($ basedir. "B. class. php ");
?>

/Classes/B. class. php
$ Basedir = dirname (_ FILE __);
?>

/C. php
$ Basedir = dirname (_ FILE __);
Require_once ($ basedir. "classes/a. class. php ");
?>
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You can use an absolute path.
No problem with the introduction.