John Cena - no offense, but have you ever done programming before? If so, then A) you'll understand the difference between a programming language and a program built in that language, and 😎 you'll understand that languages themselves don't have "themes".
phpBB is a free, open-source forum/message board/BBS/etc., written in the PHP language. CMS is an acronym for Content Management System. It's basically a system (written in any language, but in your case, PHP) which manages content for a site. For example, a site like a newspaper will use a CMS. The CMS presents forms for the users (staff) to fill out with the articles, etc., and the CMS takes care of updating stuff to the database, displaying it to end-users, taking care of searches, etc. It's easier than writing one HTML page for every single article, and it promotes data consistency.
A BBS is an acronym for Bulletin Board System, and an example of one is phpBB. It's basically the same thing as a forum or message board.
"its a PHP based website theme. so can i install it on my local pc??"
Now, that you understand more about PHP, you'll understand that there isn't anything as a "PHP-based website theme". Perhaps it's a theme for a particular CMS/BBS; if so, you'll need to download & install the correct program. If not, then it might be a regular theme for a website, but it utilizes PHP for something (either dynamic date generation, visitor logging/tracking, etc.). Post some code and we may be able to help you.
In any case, you will need to install (at a minimum) PHP, Apache (or IIS), and MySQL. I also reccomend installing phpMyAdmin to complete the "LAMP" environment (even though phpMyAdmin isn't technically part of LAMP...well, now I'm just rambling 😃)
