PHP web developers create websites which are dynamic, meaning they can store, receive and display variable information to the users of the website. PHP is a free open source language and is the most commonly used scripting language on the web.
It enables the visual front end, to communicate with a backend database and pass information back and forth. It typically runs on a linux operating system, with an apache server and MySql database. PHP developers can also choose to run it in a windows server environment or connect to databases other than MySql database.
For a basic website which is used for promoting a business, service or person which requires a few pages, then html would suffice, although php may be useful for creating a contact form.
A PHP developer would use php on a larger website which contains a large amount of information. For example for a job website with thousands of jobs, it would be totally impractical to maintain by hand. Therefore typically the employer with the vacancies, would be able to register their information and post vacancies via a form on the site.
The information contained in the form, once submitted by the user, is automatically entered into the database and stored. A user looking for a vacancy, can make a search via a front end php page, then if their search criteria matches a vacancy held in the database, then this information can be displayed to the user via the php page. That is just the start of what php can do for your website project.