processing radio buttons from a form

[cre8nkos]

I'm doing an api call to my credit card processor and they get the data via xml. I need to have radio button options in my form, and need to check to see which one is checked to know what value to send to the credit card company. Right now I have:
if ($_POST["chargetotal"]=="mailorder")
{
$myorder["chargetotal"] ="29.99";
}
else
{
$myorder["chargetotal"]="19.99";
}
but it says "invalid xml" when I submit the form. Am I reasigning the values wrong? I'm new to PHP - obviously.

Also, I get": Warning: main(.:/usr/lib/php:/usr/local/lib/php) [function.main]: failed to open stream: No such file or directory in /home/pondowne/public_html/pages/PHP_contactform.php on line 35

despite the fact that I do have this included in the file.

Thanks in advance for the help!
Amy