I'm designing an iOS app could help us in game. The app including a "spell quick reference" section. Can I use the spell list in the PHB? In the future, I want to add more to it, like feats. Can I use them in my app?
But you can use the content in the SRD (Systems Reference Document), as long as you follow the terms of the OGL (Open Gaming License), which is included with it.
The SRD contains all the mechanical rules and some of the creative content from the PHB, MM, and DMG.