Help with PHP and checkboxes

Hi,

I'll explain basically what I'm trying to do.

A person goes to my site to download. Before they can download they have to agree to the TOS (according to my lawyer this is best). So on the page (let's call it agree.php) I need several things:

1. textarea with the TOS. This is no problem. I know how to do that.

2. a checkbox stating I have read and agree to the TOS. No problem input type=checkbox.

3. Validation. I can do client-side no problem (using JS) but I'm after server side validation.

Can someone tell me how to do this with php or recommend a good resource? I don't know much PHP or this would probably be simple. Here was my best guess:

<?php


/* Get URL to "goto" from POST var */


$agr = $_POST["agree"];


if ($agree == checked){
header("Location: http://www.downloadurl.com/");
}


elseif ($url != checked){
header("Location: http://www.nodownloadurl.com");
}


?>

Also if possible I was hoping to simple have the PHP write in at the top of the page 'You must agree to TOS' if they tried to submit without checking the box. Is that possible? Solutions and resources are appreciated.

I tried using the above, but it instantly redirected instead of waiting for the form to be submitted. :lol:

Greg