I'm using the Tribulant Checkout shopping cart plugin on a client's site: http://tribulant.com/ When we tested making purchases through the cart we found the emails the cart sent and the pdf receipts where blank/white.
After much trial and error I discovered that when your "Insert PHP" plugin was active the receipts/pdfs were blank but deactivating your plugin then they worked perfect.
I do not know why these two plugins conflict... it doesn't seem like they should... but they do. I have let the Tribulant support team know of the issue.
For now, I have rewritten all the pages were I was using your plugin so as not to need it anymore. But I wanted you to know about this anyway.
BTW, it worked great except for this conflict.