How to Synchronize POS system with Inventory in an online store

I have this client requirement to integrate their inventory(which using for online clothing store developed in PHP) with the POS system(has an API). My question is, how can I keep consistency in these two systems while processing orders. Is it better to use realtime POS and inventory updates on each order or scheduled synchronization? Please excuse me and tell me if the question is not clear. Detailed guidance appreciated. Thanks