Account, inventory integration with existing web application.

Question

I have a midsize ERP which is very much domain focused.

Now, I need to have typical ERP features like Accounts, inventory, purchase etc etc. I see a lot of applications already available for these.
So I am thinking of reusing any of the existing available package and integrate it with my app.

So the question is : Can anyone refer me such an application.

Further insights ;

1. My app is LAMP based (Linux apache Mysql PHP)
2. I would prefer a commercial opensource application
3. The application should have extensive api for integration
4. The application must be web based.