In a new post to the CatsWhoCode.com blog Jean-Babtiste Jung walks you through the creation of a simple monitoring script written using just PHP that can tell you if your web site is up and responsive.

In order to make sure that your website is always available to the public, you have to monitor it. In this tutorial, I’ll show you how you can easily create a monitoring script that will check your website availability and send an email or sms alert to you if it isn’t.

They have a snippet of code (about 15 lines long) with a function you can call to check a remote host's connection and check the returned data for a certain string. This can not only ensure that your site is responsive but also that it's not responding incorrectly. You could even use this to hit a certain monitoring page of your site to check for certain things (like database connection problems).