PHP snippet prints to a white page

I've created a webform that redirects to an internal node (page) that has a php included into the body. That node takes the form values and calls a php function from the included script. When I print anything from the php script, it prints that output to a white screen, and I get

Cannot modify header information - headers already sent by (output started at testPurchase.php:48) in /var/www/drupal/includes/common.inc on line 319.'

I have read through the snippet guide, and was sure to remove all whitespace from the embedded code before and after the

, as well as only using one code block. How do you return values from the embedded script without blowing away the layout?