PHP provides a powerful language with file operation commands, image manipulation commands, text processing commands, database connection commands, and many others.
PHP code is run on the server before the code is sent to the user and so the end user never sees the PHP code. All they see is the HTML that the PHP code produces. This can be used to process passwords and other secure code where you don't want the end user able to see everything that is done behind the scene.
When you click a link, the client sends any relevant information to the server along with any cookies for that site. The PHP code compiles any includes, makes any necessary calls to the database, and creates a HTML page to send back to the user, along with any javascript or CSS files.
Here is the life cycle of a web request to a PHP document:
PHP files can include HTML and the easiest way to get started is to make a file that is mostly HTML and just add a little bit of PHP at a time between <?php and ?> tags.
<html>
HTML code goes here
<?php
PHP commands;
?>
more HTML
<html>
On the syntax page I include many of the commands and how to use them.
PHP is a great subject. You can always learn more about it.