Question: I have designed a website in php language. I have a search box in my website. Now i want to make it usable means that if a user wants to search anything he could just type there and can get the desired result. Is this necessary to use php or it could be happened easily with HTML tags???

Answer: 
Easiest is to add Google (or another search engine) site search. You won't need PHP, but a bit of HTML is needed. 

[Google Site Search](http://www.google.com/cse/)

While google site search is the easiest solution you can also use your own PHP search solution or use a 3rd party script such as this one

[3rd party search spider script](http://www.hotscripts.com/listing/zoom-search-engine-add-search-to-your-site-php/) (I haven't used it myself nor affiliated, it's an example of a type of script that you might find on the interweb and not a recommendation)
the best thing is that you should use google search
implementing google search is not that hard. you have to provide information about your site and a script will generate which you have to put on your html page