Scaling a PHP application to handle increased traffic and maintain user experience is crucial for businesses. The two main methods of scaling are vertical scaling, which involves increasing system resources, and horizontal scaling, which involves adding more servers to an existing cluster. Horizontal scaling is generally considered the most reliable and efficient method, but it requires careful planning to keep all nodes in the cluster updated and synchronized. This includes implementing sticky sessions, using a load balancer to distribute traffic, and setting up a replica of the application on multiple nodes. Additionally, it's essential to decouple the application from the database server, handle user session consistency, and implement a solution for sharing files uploaded by users. Tools like Memcached and Redis can be used to handle user sessions, while GlusterFS or object storage services like AWS S3 and Google Cloud Storage can be used to share files. With proper planning and implementation, horizontal scaling can help PHP applications maintain performance and scalability even in high-traffic environments.