UI development in PHP involves building dynamic and interactive interfaces by combining PHP with front-end technologies like HTML, CSS, and JavaScript. PHP acts as the backend, processing user inputs, handling forms, and managing sessions while dynamically generating the content to display on web pages. Using PHP frameworks like Laravel or CodeIgniter helps separate logic from the interface, enhancing UI design through templating engines like Blade or Twig. Integrating PHP with JavaScript libraries (e.g., jQuery, Vue.js) can create more responsive, interactive user experiences, while CSS frameworks (e.g., Bootstrap) ensure mobile responsiveness and design consistency across the application.