One typical and very powerful use of PHP is to personalize sales pages. For example lets say you have a squeeze page (explained in sales process section) offering a free demo of your product. When the user enters their name and email address into the form on the squeeze page you can have a php script(program) that reads the persons name and then uses it on any following pages that they see. When done well this is very impressive to your website visitor.
PHP is also a great tool to use to interact with a database on your website. A simple example would be where you have a membership website that requires members to log in every time they use the website. The members username and password are stored in the database and the login page uses a php script to check that the username and password are valid.