My wife's visa was unexpectedly denied. She is traveling on Singapore Airlines, Philippines to the UK. I paid for the tickets in full, as I didn't expect any problem. Can I get a refund? Or change the flight dates if our appeal is successful?
As pointed out in the other answer, there's no refund due to refused visas, and no airline I know of except Ukraine International provides for this.
You can only get the airport taxes refunded.
Also UK visa refusals cannot be appealed, you can only make a fresh application with evidence addressing the reason for refusal. You may wish to ask a separate question on here about this, uploading her refusal letter.
There is no exception in the Singapore Airlines Contract of Carriage which allows for an exceptional refund or change of flight details should the traveler be refused a visa.
There is also no legal requirement for Singapore Airlines to issue a refund in this instance.
You should either not book flights until you receive the visa, or if flights are required to be booked by the visa issuing authority then you should book the more expensive refundable/changeable fares.