Check if a request is from Facebook Crawler

fb.php

<?php

function check_facebook() {
	$IP = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : false;
	$UA = isset($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : false;
	if (!$IP || !$UA) return false;

	$UAs = ['facebookexternalhit', 'Facebot', 'visionutils'];
	$UAs = array_map('preg_quote', $UAs);
	if (!preg_match('#^' . implode('|', $UAs) .'#i', $UA)) return false;

	$output = [];
	$command = 'whois -h whois.radb.net -- ' . escapeshellarg('-K ' . $IP);
	exec($command, $output, $return_var);

	if ($return_var !== 0 || count($output) !== 2) return false;
	if (!preg_match('#^origin\:\s+AS32934$#', $output[1])) return false;

	return true;
}