Detect when a website changed by logging hashes of the content (eg. check for domain availability)

check_website.php

<?php

/*
Since this script works by detecting changes to the page source code
it works only on static pages. Sites that often change, eg. are generated
by a CMS or embed changing script filenames will always trigger.
There is no easy way around this limitation as far as I know.

Instructions:
- Add your Email and desired URLs
- Run the script periodically (I set up a cron job: 0 7 * * * /usr/bin/php ~/check_website.php)
*/

// settings
$notify_email = 'your.email@host.com';
$domains = array(
	'http://domainnametowatch.com',
	'https://somepage.net/dir/site.html'
);

// load existing data
$script_filename = basename(__FILE__, '.php');
$log_filename = __DIR__.DIRECTORY_SEPARATOR.$script_filename.'.json';
$log_file = '[]';
if(file_exists($log_filename)) {
	$log_file = file_get_contents($log_filename);
}
$log_data = json_decode($log_file, true);

$now = time();
$messages = '';
foreach ($domains as $domain) {
	$hash = hash_file('sha256', $domain);
	$headers = get_headers($domain);

	$found = false;
	if(sizeof($log_data) > 0) {
		foreach ($log_data as $log_item) {
			if($log_item['domain'] == $domain) {

				if($log_item['hash'] !== $hash) {
					// site content changed
					$messages .= 'The content of the page at '.$domain.' changed from '.$log_item['hash'].' to '.$hash.'.'.PHP_EOL;
				}
				
				if($log_item['status'] !== $headers[0]) {
					// http status response changed
					$messages .= 'The http status of the page at '.$domain.' changed from "'.$log_item['status'].'" to "'.$headers[0].'".'.PHP_EOL;
				}

				$found = true;
				break;
			}
		}
	}
	
	if(!$found) {
		$log_data[] = array(
			'domain' => $domain,
			'hash' => $hash,
			'status' => $headers[0],
			'last_check' => $now
		);
	}
}

// log results to file
$json_data = json_encode($log_data, JSON_PRETTY_PRINT);
file_put_contents($log_filename, $json_data);

// notify via email
if(strlen($messages) > 0 && !empty($notify_email)) {
	@mail($notify_email, 'A watched website changed!', $messages);
}

Just a wild idea: How about initializing a local git and commit each change locally to detect changes?:) (through executing git-diff / git-status etc over php)