You can send messages between WhatsApp and Slack using Twilio and CakePHP by setting up a Slack API app, creating a new CakePHP application, installing the Twilio PHP Helper Library, configuring the Slack app webhook settings, storing the application's configuration settings, connecting to the Twilio WhatsApp Sandbox, creating the application's core controller, setting up the application's route, and setting up a Twilio WhatsApp webhook. The core controller receives incoming messages from WhatsApp and sends them to the nominated Slack channel. To test the application, you need to send a message from your WhatsApp number to the Twilio number and verify that the message is sent successfully to the Slack channel.