With php you can make it easier to add content in the future by using MySql and Php to display data from a database. But for a site with few pages it is not smart to use php unless you have a user interface to add data or contact you.
You can use PHP includes to make editing your content and templates easier, or if you need something a little more advanced you can look in to a content management system. There are hundreds out there so you'd need to do a fair amount of research to decide which one suits your needs - it depends on the type of website you are running.