Document tree and PHP extension problems

The document tree and the PHP extension are found in the book PHP and MySQLWeb development. this text is not easy to understand. I hope the experts can answer this question. & nbsp; PHP does not view the required file extension. This means that you can name the file without directly calling the file. When a file is loaded using the require () statement, it will be used as a PHP file document tree and PHP extension
In the book "PHP and MySQL Web Development", I am not very familiar with such a paragraph. I hope you can answer it.
PHP does not view the required file extension. This means that you can name the file without directly calling the file. When a file is loaded using the require () statement, it is executed as part of the PHP file.
Normally, php sentences are not processed if they are placed in an HTML file (for example, page.html. PHP is usually used to parse files with extensions defined as. php. (This can be the same in the webserver configuration file. However, if this page.html file is downloaded through require( syntax, any PHP commands in the file will be processed. Therefore, you can use any extension to name the inclusion file, but try to follow a convention, such as naming the extension. inc is a good method.
Note that if the extension is. inc or some other non-standard extension files are stored in the Web document tree, and users can directly load them in the browser, users can view the source code in the form of common text, include any password, so it is important to save the included files outside the document tree or use the standard file extension.

As mentioned in this article, it is a good way to name the extension as. inc. why? Secondly, what is the document tree mentioned in it? Finally, how to save the contained files outside the document tree

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This means to prevent the contained php file from being out of the website document directory and ensure that apache running users have the right to access this directory.

For example, if the default directory of apache is/var/htdoc, you can name the included PHP file. the suffix of inc is placed in the/var directory, so that the browser cannot access the files under/var.