PHP
In most of the case we need our client can easily connect with merchant in ecommerce and one of the best way is to connect by social media among then whatsapp is one of the popular one.
We can achieve this by below given whatsapp API
For Mobile
whatsapp://send?l=en&phone=xxxxxxxxxx;
For Web:
https://web.whatsapp.com/send?l=en&phone=xxxxxxxxxx;
Below give github code is extension for magento1
gitHub Code: https://github.com/durgagupta/magento1-whatsapp-chat/tree/master