A simple call-home alive check script based on server-side PHP and client-side curl/cron

alive.php

<?php

/**
 * A simple call-home alive check script based on server-side PHP and client-side curl/cron
 */

error_reporting(0);
ini_set('display_errors', 0);
date_default_timezone_set('Australia/Melbourne');
header('Content-Type: text/plain');

// Define host keys
const HOSTS = [
	"00000000-0000-0000-0000-000000000000" => "My Router",
];

const FLAG_TIMESTAMP = "/tmp/alive_checkin_%s.flag";
const FLAG_DISCONNECTED = "/tmp/alive_disconnected_%s.flag";

/**
 * Send PushOver notifications (without cURL extension)
 * 
 * @param string $title Title of the notification
 * @param string $message Notification body
 * @param string $priority Message priority, refer to https://pushover.net/api#priority
 */
function pushover($title, $message, $priority) {
	$result = @file_get_contents(
		"https://api.pushover.net/1/messages.json",
		false,
		stream_context_create([
			'http' => [
				'method' => 'POST',
				'header' => 'Content-Type: application/x-www-form-urlencoded',
				'content' => http_build_query([
					"token" => "-----PO_APP_TOKEN-----",
					"user" => "-----PO_USER_KEY-----",
					"title" => $title,
					"message" => $message,
					"priority" => $priority,
				])
			]
		])
	);
}

/**
 * Create human readable time up to hours
 * 
 * @param int $time Number of seconds to parse
 * @return string Human readable time (e.g. 23 hours 15 minutes and 1 second)
 */
function time_readable($time) {
	$hours = floor($time / 3600);
	$readable_hours = $hours . " hour" . (($hours > 1) ? "s" : "");
	$minutes = floor(($time % 3600) / 60);
	$readable_minutes = $minutes . " minute" . (($minutes > 1) ? "s" : "");
	$seconds = ($time % 3600) % 60;
	$readable_seconds = $seconds . " second" . (($seconds > 1) ? "s" : "");

	$readable = "";

	if ($hours > 0) {
		$readable .= $readable_hours;
		if ($minutes > 0 && $seconds > 0)
			$readable .= " ";
	}

	if ($minutes > 0) {
		if ($hours > 0 && $seconds == 0)
			$readable .= " and ";
		$readable .= $readable_minutes;
	}

	if ($seconds > 0) {
		if ($hours > 0 || $minutes > 0)
			$readable .= " and ";
		$readable .= $readable_seconds;
	}

	return $readable;
}

/**
 * Process home call requests
 * 
 * @param string $hostkey Host key of the monitoring target
 */
function action_ping($hostkey) {
	$timestamp_file = sprintf(FLAG_TIMESTAMP, $hostkey);
	$disconnected_file = sprintf(FLAG_DISCONNECTED, $hostkey);

	$last_checkin = intval(@file_get_contents($timestamp_file));

	// Write current timestamp as checkin time
	file_put_contents($timestamp_file, time());

	// Remove flag and notify on reconnection
	if (file_exists($disconnected_file)) {
		unlink($disconnected_file);

		$offline = time() - $last_checkin;
		pushover(
			sprintf("%s is back online", HOSTS[$hostkey]),
			sprintf(
				"%s is back online. It was offline for\n%s",
				HOSTS[$hostkey],
				time_readable($offline)
			),
			0
		);
	}
}

/**
 * Process check alive requests
 * 
 * @param string $hostkey Host key of the monitoring target
 */
function action_check($hostkey) {
	$timestamp_file = sprintf(FLAG_TIMESTAMP, $hostkey);
	$disconnected_file = sprintf(FLAG_DISCONNECTED, $hostkey);

	$last_checkin = intval(@file_get_contents($timestamp_file));
	$last_checkin_readable = date('r', $last_checkin);

	// Check if last checkin is < 60 seconds
	if (time() - $last_checkin > 60) {
		if (!file_exists($disconnected_file)) {
			// Create flag and notify on first disconnection
			file_put_contents($disconnected_file, "disconnected");
			pushover(
				sprintf("%s is offline!", HOSTS[$hostkey]),
				sprintf(
					"%s seems to be offline.\nLast check-in: %s",
					HOSTS[$hostkey],
					$last_checkin_readable
				),
				1
			);
		}
	}
}

// Test if parameters are all present
foreach (['hostkey', 'action'] as $query) {
	if (!array_key_exists($query, $_GET)) {
		http_response_code(422);
		die();
	}
}

// Test if host key is valid
if (!array_key_exists($_GET['hostkey'], HOSTS)) {
	http_response_code(403);
	die();
}

// Process actions
switch($_GET['action']) {
	case 'ping':
		action_ping($_GET['hostkey']);
		break;
	case 'check':
		action_check($_GET['hostkey']);
		break;
	default:
		http_response_code(403);
		die();
}

client.crontab

# Call server every minute
* * * * * /usr/bin/curl -m 10 "https://api.billgong.com/alive?hostkey=00000000-0000-0000-0000-000000000000&action=ping" >/dev/null 2>&1

server.crontab

# Server-side check client last check time - better wait a few seconds before checking in case the old timestamp exceeds the 60 seconds threshold
* * * * * sleep 15;/usr/bin/curl -m 10 "http://api.billgong.com/alive?hostkey=00000000-0000-0000-0000-000000000000&action=cron" >/dev/null 2>&1