how can i get search word when a visitor comes from google search to my site?

[timberlandoutlet4cdf]

I want to customize my index.php when a visitor searchs for a word in google search and comes to my site.ex:\[code\]if(visitor searches for "A" in google search) do somthing;elseif(visitor searches for "B" in google search) do somthing else;\[/code\]if i should use referrer tell me how?