How does PHP obtain the search engine and keyword of the inbound website? PHP Tutorial

How does PHP obtain the search engine and keywords of the inbound website. Sometimes, when we enter a website from a search engine, there will be a line of small words, "Welcome from Google Baidu, by retrieving the" XXX "keyword to this site" and so on. So sometimes, when we enter a website from a search engine, there will be a line of small words, "Welcome from Google/Baidu, by retrieving the" XXX "keyword to this site" and so on. How can this function be implemented? In fact, it is not difficult. the general idea is to obtain the inbound URL and repeat the data we need.

 0) {$ start = stripos ($ url, '&'); $ s_s_keyword = substr ($ url, 0, $ start );} else {$ s_s_keyword = substr ($ url, 0);} return $ s_s_keyword;}?>

Bytes. So this...