Even though a website built with PHP can have hundreds of thousands of lines of code, websites do not rely on PHP alone. PHP scripts generate HTML pages which often have also javascript embedded. There are javascript libraries like Mootools, Scriptaculous and jQuery which simplify the creation and manipulation of HTML documents.
You could first familiarise with the concept of MVC - Model, View and Controller, to see how websites can be structured. From there it is an easy step to install a Content Management System (CMS) like Joomla and get a flying start in building sites. Joomla has hundreds of extensions which are free. You could run a site with a photo gallery, chat room and support forum in no time, using the application from the Open Source communities. In due course, you can return the favor and publish something you have developed.
It is a huge area, and lots of fun. Even with lots of experience in IT in general, I learn new things every day :-)