Facebook Crawler Request Rate Limiter

fblimit.php

<?php

// Number of requests permitted for facebook crawler per second.
const FACEBOOK_REQUEST_THROTTLE = 20;
const FACEBOOK_REQUESTS_JAR = __DIR__ . '/.fb_requests';
const FACEBOOK_REQUESTS_LOCK = __DIR__ . '/.fb_requests.lock';

$ua = $_SERVER['HTTP_USER_AGENT'] ?? false;
if ($ua && strpos($ua, 'facebookexternalhit') !== false) {

	$jar = @file(FACEBOOK_REQUESTS_JAR);
	$currentTime = time();
	$timestamp = $jar[0] ?? time();
	$count = $jar[1] ?? 0;

	if ($timestamp == $currentTime) {
		$count++;
	} else {
		$count = 0;
	}

	file_put_contents(FACEBOOK_REQUESTS_JAR, "$currentTime\n$count");

	if ($count >= FACEBOOK_REQUEST_THROTTLE) {
		header("HTTP/1.1 429 Too Many Requests", true, 429);
		header("Retry-After: 60");
		die;
	}

}

// Do other stuff...