确保paypal购买表格未被编辑

so my question is how i make sure the paypal form isnt going to get edited by a user on my website.

the form tags are of the "hidden" type but a quick look at the source code lets you change that.

if a line of the form is:

<input type="hidden" name="amount" value="71">

how can i make sure someone doesnt change it to

<input type="hidden" name="amount" value="1">

and pay 1 dollar for the item, since paypal is going to process his request.

is there a way to post a form like this using PHP where the end user cannot change its parameters on the fly? its really important. if people would attempt fraud then i wouldnt care but im very sure someone would try to do thus.

thank you for all the suggestions, and sorry if its not a very specific "question"

could i use this maybe?

http://www.html-form-guide.com/php-form/php-form-submit.html