PHP Simple Uptime Monitoring. Can be used in cron or command line.

check.php

<?php 
error_reporting(E_STRICT | E_ALL);

$url_to_check = 'http://domain.zyz';

$downtime_message = 'WEBSITE IS DOWN : '. $url_to_check;
$uptime_message = 'WEBSITE IS BACK ONLINE : '.$url_to_check;
$notify_email = 'me@domain.xyz';
$from_ok = 'UPTIME OK';
$from_error = 'UPTIME ERROR';
$from_email = 'uptime-monitor@domain.xyz';
$check_text = '<!-- secret text in website -->';

$error_file = 'upmon-error.txt';
$ok_file = 'upmon-ok.txt';

date_default_timezone_set('Etc/UTC');

// requires PHPMailer
require 'PHPMailerAutoload.php';

$mail = new PHPMailer;
$mail->isSMTP();
$mail->Host = 'email-smtp.us-east-1.amazonaws.com';
$mail->SMTPAuth = true;
$mail->SMTPKeepAlive = true; // SMTP connection will not close after each email sent, reduces SMTP overhead
$mail->Port = 587;
$mail->Username = 'AKIAIAR6CCUUABCDEFIQ';
$mail->Password = 'Ana0bkUG/gYCaVTi+PHs8jUS8VsE89V0Fgqhj123456';
$mail->addAddress($notify_email);

$webhtml = file_get_contents($url_to_check);

if ( strpos($webhtml, $check_text) === false) {
	
	@unlink($ok_file);
	$error_time = @file_get_contents($error_file);
	$nowtime = time();
	
	if($error_time === false) {
		file_put_contents($error_file, $nowtime);
		$error_duration = 0;
	} else {
		$error_duration = $nowtime - $error_time;
	}
	
  // if you use this in cron, you will continue to get email 
  // while the website is still down.
	$mail->setFrom($from_email, $from_error);
	$mail->Subject = $downtime_message . " Duration : ".sectime($error_duration);
	$body = $url_to_check . " has been down for ".sectime($error_duration);

	$mail->msgHTML($body);
	if (!$mail->send()) {
		echo "DOWN Mailer Error\n";
	} 	
	echo $url_to_check . " has been down for ".sectime($error_duration);."\n";
} else {
	
	@unlink($error_file);
	$ok_time = @file_get_contents($ok_file);
	$nowtime = time();
	
	if($ok_time === false) {
		file_put_contents($ok_file, $nowtime);
		$ok_duration = 0;
		
    // if the website was down, you will get an email when it is back online
		$mail->setFrom($from_email, $from_ok);
		$mail->Subject = $uptime_message;
		$body = $uptime_message;
		
		if (!$mail->send()) {
			echo "UP Mailer Error\n";
		} 			
		
	} else {
		$ok_duration = $nowtime - $ok_time;
	}
	
	echo $url_to_check ." OK Duration : ".sectime($ok_duration)."\n";
}

function sectime($seconds) {
 $t = round($seconds);
 return sprintf('%02d:%02d:%02d', ($t/3600),($t/60%60), $t%60);
}