Making a PHP website communicate to Dolibarr ERP/CRM should best be done by either using the Dolibarr REST API or developing custom modules. This allows the ability to get, update, and manipulate data of the customer, invoices, products, and others, coming from an external PHP website. It supports API keys for authentication, and you make calls to interact with the system. If you have specific business logic to implement or integrate custom logic, then you could create a PHP module to directly interact with the database of Dolibarr.