From http://stackoverflow.com/questions/1805296/get-keyword-from-a-search-engine-referrer-url-using-php

providergoogle.php

// The referrer
$referrer = 'http://www.google.com/search?hl=en&q=learn+php+2&client=firefox';

// Parse the URL into an array
$parsed = parse_url( $referrer, PHP_URL_QUERY );

// Parse the query string into an array
parse_str( $parsed, $query );

// Output the result
echo $query['q'];