隐藏真实背后的机制是什么？

According to this "Cloaking refers to the practice of presenting different content or URLs to human users and search engines" The same link gives example as:

Serving a page of HTML text to search engines, while showing a page of images or Flash to users

Question: If I had correctly interpreted, there must be a mechanism for identification of the entity whether it is search engine or browser(user) at the web server.What do we call such mechanism? Or is it just a php or JavaScript code that redirects? How does a web-server actually know that entity'X' search engine and entity'Y' is web browser?

The user agent is a good way to identify the client.

This is the user agent string passed to the server on a request from a browser:

"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36"

From Google:

Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

From Bing:

Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)

To use them with PHP, you may do something like this:

if (strpos($_SERVER['HTTP_USER_AGENT'],'bot') !== false) {
    // This is probably a bot
}

If you want to be a little more precise, you might want to also check for a link, like so:

$userAgent = $_SERVER['HTTP_USER_AGENT'];
if (strpos($userAgent,'bot') !== false && strpos($userAgent,'http') !== false) {
    // It is probably a bot
}

This question and answers show how to use Apache to deliver different content based on user agent: Rewrite rule for user agent with mod_rewrite