If I were to send a heart emoticon from my Android phone to an iphone what would the iphone user see?

This is just a guess, but since the symbol you send is HTML code, I think the iPhone recipient sees the same thing.

That sort of thing is often application-dependent, so it may be <3, or maybe ♥, or maybe their SMS app has it’s own graphic.

We just ran a test.

From his Galaxy S4, my sweetie sent <3 via the native Messaging app and his display showed the ‘green blob’ (which looks to me like the Android logo robot’s head with hearts for eyes), and my iPhone 5S received <3. I responded with :-*, and the Android Messaging app displayed the robot’s head with kissy-lips. This is one of the reasons that he doesn’t use that as his default messaging app.

Instead, he uses the Hangouts App as his default messaging app, from which he can send to either my native iOS messaging app, or my Hangouts for iOS app. With both of those, he and I both saw <3 and :-*.

However, in our natural day-to-day interactions, we use emoji; which appear the same on both devices.

Emoji are a bit different in that they actually have an internationally accepted standard that they adhere to rather than some proprietary stuff that no other program is really obligated to understand.