Do you think this is just sunburn - or an infection?
 

Do you think this is just a sunburn or an infection? It was indoors all winter and is now outdoors on a screened porch.

Looks like sunburn. Phals. don't ever need any sun on their leaves.

Could also be heat. Orchids don't like sudden change. They can acclimate to higher light (and warmer weather) if transitioned gradually. But ES is spot-on, Phals don't like direct sun ever, even for 5 minutes. It looks to me like that leaf just got toasted. Since it is an old leaf, it might have been on the way out anyway. Also, note that if light and temperature increase, water needs to increase too - you want to make sure that it doesn't get dehydrated, with higher temperature water evaporates faster.