PayPal Returns to Shopping Cart Page

Just today I had a customer say that he wasn't able to complete a purchase with PayPal. I did a test purchase. The checkout was normal all the way through the PayPal site. However, while on the PayPal site, when I clicked to submit my payment information, I landed on the shopping cart page with an empty shopping cart. The test order also didn't show up in admin.

I didn't receive any error messages, so I can't provide any information other than what I just experienced. I haven't made any changes to the PayPal module or to my PayPal account. However, on Thursday, I adjusted some settings in my php.ini file. But I received PayPal orders since then, except for today. Credit card orders are also coming through fine.

Would this be related to my php.ini settings or something else altogether? I'm at a lost as to where the problem could be.

Never mind. I just figured out that it was a problem with sessions cookie. I forgot that I had adjusted that setting in my php.ini file earlier this morning. Once I commented them out, my test purchase went through.

;session.cookie_domain = "mydomain"
;session.referer_check = "mydomain"

Regarding these specific settings, should I just leave them alone, or is there another way to secure cookies without messing up PayPal transactions?