Looks up the user agent against a known list, and outputs a HTML comment

userAgentSEO.php

<?php

// Quick script to check and output the user agent in a HTML comment
// Jono Alderson [https://www.jonoalderson.com]
// =================================
// Google user agent strings are sourced from
// [https://support.google.com/webmasters/answer/1061943?hl=en]
// Feel free to add/edit definitions
// =================================
// Last updated: 20/03/2018


// Housekeeping & setup
// =================================
if (!isset($_SERVER) || empty($_SERVER)) { return false; }
if (!isset($_SERVER["HTTP_USER_AGENT"]) || $_SERVER["HTTP_USER_AGENT"] == false) { return false; }
$userAgent = $_SERVER["HTTP_USER_AGENT"];

$userAgents = array(
 "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" => "Google (Desktop)",
 "Mozilla/5.0 (Linux; Android 6.0.1; Nexus 5X Build/MMB29P) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.96 Mobile Safari/537.36 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" => "Google (Mobile)"
);


// Check if the string matches a known agent
// =================================
if (in_array($userAgent,$userAgents)) {
 $userAgentString = $userAgents[$userAgent];
} else {
 $userAgentSanitized = htmlspecialchars($userAgent);
 $userAgentSanitized = str_replace(array("-->","<!--"), "", $userAgentSanitized);
 $userAgentString = "OTHER - " . $userAgentSanitized;
}


// Construct the comment
// =================================
$date = date('r');
$comment  = "<!-- " . PHP_EOL;
$comment .= "User agent: {$userAgentString}" . PHP_EOL;
$comment .= "Time: " . $date . PHP_EOL;
$comment .= "-->". PHP_EOL;

echo $comment;

How do I use this code please ?