Widgetwhats.com is the best solution for your website to have a WhatsApp widget. You can create your own script and include the WhatsApp Window in your website with PHP.
There are plenty of solutions base on PHP to create websites. The issue is with PHP there is not a marketplace for modules. Widgetwhats.com offers a solution to create a WhatsApp Chat bot for your PHP website.
The script we generate should be included in your header.