PHP works in paraelle / conjunction with HTML so essentially you will use it along side it, not "develop a new site using php".
If you're using a "Hosting" provider, then, they will tend to have PHP/MYSQL installed on their servers, however, if you are using your own computer then you can download apache/php/mysql .. If this is the case, what operating system are you working on?
Let me know what are the basic needs to upgrade my website development
PHP lets you program dynamic websites. You'll need to know HTML and CSS. You should be familiar with C syntax. If you want database-driven applications, you need to be familiar with Structured Query Language (SQL), and a database management system (DBMS) such as MySQL. You need a server that runs PHP.
You should become familiar with php.net, as it has a great manual and documentation for the language. Tutorials for codings in PHP are available all over the Web.
Basically you need to learn
1. This one is for UI (user interface)
you can embed PHP to HTML(note: you must be familiar with C language or any other related programming languages like Ruby etc...if you know any other programming languages it'll be easy for you)
you'll need to have a server/Apache (XAMPP/WAMP for Localhost) and of course if you're going to create system that requires a database. probably you need to learn MySQL or any other available databases.
Read tutorials over the net or from books (But i prefered over the net, books might get outdated someday)
I agree with the others: its easier learning when you work it into an existing site...