Escaping JSON values

[LeftPinkie]

I am using PHP SDK "authorizenet/authorizenet", version 2.0. Today I received error "E00013 Extra Options is invalid" when I went to charge a card "create a transaction API". I found out the issue was that I had an ampersand in the PO # even though the documents states that the poNumber field is a JSON string. The solution or work-around is to escape the ampersand character.

A few questions:

1. why would a JSON-encapsulated string field cause this error?
2. Are there any other string fields that would need escaping?
3. The "E00013 Extra Options is invalid" is misleading and not accurate.