So I'm making this basic 2D engine using Haxe Kha, and i just need to integrate some physics. Just basic collision detection and gravity on basic shapes, nothing too detailed. But i want to try not to write it myself, so i just want suggestions on what open source phyisics engines i could use, and that wont be impossible to integrate. Thanks in advance.

You could look into other peoples game engines that use Kha, take a look at how they handle physics in their engines and what external libraries or plug-ins they use. You could always use some of their physics code in your engine.  

Heres a few you could use with Haxe  

Nape Physics(2D): 

http://napephys.com/

Physaxe(2D): 

https://github.com/ncannasse/physaxe

Cross-Platform 3D Physics Engine 

https://github.com/saharan/OimoPhysics