why would the included content send to client as response body when including a no-php file using _include_

why would the included content send to client as response body when
including a no-php file using _include_

here are two files:
index.php:
<?php
include 'index.txt';
?>
index.txt
Hello, Cifer.
when requesting index.php the browser will shows 'Hello, Cifer', what to
my confusion is the mechanism of include, the PHP Manual says many things
about include but does not point out that the include will cause the
included content to be sent to client as response body when the included
content are not closed by <?php ?>.
so, what the mechanism of include in detail?