I have been coding for almost 10 years, and a project has come along for a mobile app that I want to take on. The app will involve an iPhone/Android version of it so I am building a plan for everything now in the early stages, but of course, I would love to hear suggestions/advice to stay on the right path.

Naturally, I am thinking about building a database on a web server which will house most of, if not all of our information. Then I want to build a simple API using PHP to be able to access the info whether I am in the iPhone app, or the Android app. My train of thought tells me I can have a standard for receiving information that way from our database, and it would make it easier to build the app around that system. So far so good?

Then, my worries start here. I have read that there are DBMS specifically for Android and iPhone apps respectively. Would I use these kinds of tables on a person's local phone, to store info about their session?

Also, if I wanted to build a chat feature, I am worried that a PHP/MySQL backend would be too slow. Any suggestions for how to get optimal performance? I am thinking I want a 2-3 second delay at worst on the message transfer, but as close to real time as possible.

I might have some more questions soon, but that seems to be all I can think of right now. Really appreciate any insight I can get, been a while since I've had a nice challenge in front of me.