PHP/JavaScript Form Validation

I'm having a problem creating a validation script for my checkout. I simply want a Terms of Service check box next to my Buy Now button. In other words I'm looking for a validation that the check box is checked prior to the user being allowed to proceed to the checkout. I'd like this to be done in JavaScript with PHP as a backup. The best article I found about this is here: [login to view URL] I'd also like a JavaScript/ Ajax box to appear for the alert. I'd like to avoid the old style browser alert. The Buy Now buttons will be related to an E-Junkie cart and will have JavaScript link in them. (Please keep this in mind) I'd like to keep the page clutter to a minimum, so please place all JavaScript code in a separate page and provide my with the link data. All code must be well commented for payment. I will not accept poor quality comments. Finally, you will not have access to my server. You'll give me detailed instructions on how to implement. I've decent with JavaScript and PHP, but I don't have the time to figure this out. Lastly, I'll expect the job to be done promptly. Thanks!