Prompt Title: CRM

As an AI language model, I cannot create a fully functional WhatsApp app for you, but I can provide some guidance and examples of how you can develop a similar app using PHP and JavaScript.

WhatsApp is a messaging app that uses a combination of client-side and server-side technologies to facilitate real-time communication between users. For a SaaS-based WhatsApp app, you would need to create a web-based interface that allows users to send and receive messages.

To develop a WhatsApp-like app, you would need to create a server-side script that listens for incoming messages and stores them in a database. You would also need to create a client-side script that allows users to send and receive messages in real-time.

Here's an example of how you can develop a WhatsApp-like app using PHP, JavaScript, and the Twilio API:

1. Set up a server-side script using PHP that listens for incoming messages from users. You can use the Twilio API to receive and send SMS messages.

2. Store the incoming messages in a database for future reference.

3. Create a client-side interface using JavaScript that allows users to send and receive messages in real-time. You can use WebSockets or AJAX to achieve this.

4. Integrate the Twilio API into your client-side script to enable users to send and receive messages via SMS.

5. Use a front-end framework like Bootstrap or Materialize to create a responsive user interface.

6. Test your app thoroughly to ensure that it works as intended.

This is just a basic outline of how you can develop a WhatsApp-like app using PHP and JavaScript. You would need to customize the app to meet your specific requirements and integrate additional features like user authentication, message encryption, and multimedia messaging.

As an AI language model, I do not have access to a comprehensive list of all WhatsApp CRM SaaS apps developed for resale. However, I can suggest a few popular options that you may want to consider:

1. Twilio Flex - Twilio Flex is a programmable contact center platform that allows you to build and customize a WhatsApp CRM SaaS app. It provides all the tools and features you need to manage customer interactions on WhatsApp, including messaging templates, auto-replies, and chatbots. Twilio Flex is designed for resale, so you can brand and market the app as your own.

2. Kaleyra - Kaleyra is a cloud-based communication platform that offers a range of messaging and voice solutions, including a WhatsApp CRM SaaS app. Their platform provides features such as 2-way messaging, rich media support, and integration with third-party tools like Salesforce and Zendesk. Kaleyra's WhatsApp CRM app is also designed for resale, allowing you to offer it to your customers under your own brand.

3. Xenioo - Xenioo is a chatbot and conversational AI platform that allows you to create custom chatbots for WhatsApp and other messaging platforms. It provides a drag-and-drop interface for building chatbots, as well as features such as NLP, integrations with third-party APIs, and analytics. Xenioo's platform is also designed for resale, allowing you to create and sell chatbots to your customers.

These are just a few examples of WhatsApp CRM SaaS apps developed for resale. You may want to research other options and compare their features, pricing, and customization options before making a decision.