Which coding language is primarily used for adding interactivity to websites?

JavaScript is primarily used for adding interactivity to websites because it is a dynamic programming language that runs in the user's web browser. This capability allows developers to create responsive, interactive features such as form validation, animations, clickable elements, and real-time content updates without requiring a page reload. JavaScript can manipulate the Document Object Model (DOM), enabling it to change HTML content and styles on the fly, which enhances the user experience significantly.

HTML is used for structuring the content of web pages, while CSS is responsible for the visual presentation and layout. PHP, on the other hand, is a server-side scripting language primarily used for back-end web development. While it can generate HTML content and manage tasks like databases or server logic, it does not directly contribute to the client-side interactivity experienced by users in their browsers. Therefore, JavaScript stands out as the essential language for fostering interactivity on the web.