To use HObfus in your PHP file, you need to include HObfus.inc.php, which you installed in your website in the installation page. You can include HObfus.inc.php using the require_once function like shown below.
<?php require_once("PATH-TO-HObfus.inc.php"); ?>
where PATH-TO-HObfus.inc.php must be replaced with the actual path to hobfus/HObfus.inc.php.
To figure out the path to HObfus.inc.php, you can use a very simple PHP script like shown below.
<?php echo dirname(__FILE__)."/HObfus.inc.php"; ?>
Copy the PHP code above and put it in a text editor such as Notepad. Save it as find-path.php in the hobfus folder, which you installed in your website in the installation page.
Now open hobfus/find-path.php in your web browser. The browser will display the path to your hobfus/HObfus.inc.php like this.
/this/is/path/to/hobfus/HObfus.inc.php
Copy the path and paste it in the require_once function. For example,
<?php require_once("/this/is/path/to/hobfus/HObfus.inc.php"); ?>