PHP winding functions to get the page and if an error occurs during the connection, follow the error handling routes
Function PHP md5 to obtain the hash of the body and compare it with a good known stored value
phpmailer to perform SMTP authentication on a mail server to send the alert message.
Connect all that to a script that runs through cron and has a simple monitoring solution. Happy PHPing.
If you wanted a pre-built script, I'm sure there are some out there if you do a php site monitoring search, for example, but I can not recommend any since you do not use any of them, but rather nagios with different checks.