PHP is a Server-side language
Even though it is embedded in HTML files much like the client-side Javascript language, PHP is server-side and all PHP tags will be replaced by the server before anything is sent to the web browser.

So if the HTML file contains:

<html>
<?php echo "Hello World" ?>
</html>
What the end user would see with a "view source" in the browser would be:

<html>
Hello World
</html>