Error checking PHP - newbie needs help

I need some help setting up some error checking on my mailer.php page. Basically I want to check if the user has filled out all the required fields on the html form

http://dev-rockwood.virtualhorizons.com/about.htm#
Click on the download catalog on the right.

When I set up the error checking I want it to work like this and the mailer page would appear like this:

Please tell us your name.
Please tell us your phone number.
etc…
Use the browser back button to correct this issue.

I use this approach a lot in ASP reply pages but I don’t know enough to set up php.

Any help appreciated.