what is php used for in web design?
PHP is a general purpose scripting language that can also be embedded to HTML page. If you are writing your application PHP ..save your page with .php extension only.
What i can do with PHP?
Using PHP you can make a server side application. You can also send and receive the data from server.
You can take the input from user and also assign the access to application.
This is not it.. you can do a lot more. So, first you need to learn the PHP Scripting language, sql and HTML.
PHP is the server side scripting language that can be embedded to HTML page. By using Framework in PHP, you can use the powerful and security wise good website.