Why is it that PHP include is not adding HTML, but is able to execute document download and email sending?

edit

PHP include function is designed to include and execute PHP code within a file. It does not just add HTML content to the page. When PHP includes a file, it is interpreted as PHP code, and any PHP code within that file will be executed.

While it is possible to include HTML files using PHP include function, it is not recommended, and there are better ways to include HTML content in a PHP file, such as using the filegetcontents() or readfile() functions.

As for the document download and email sending, these functionalities are implemented using PHP functions such as header() and mail(). These functions are designed to execute specific tasks and do not require the inclusion of HTML content.