Why can't PHP maintain a long connection when using Sever-Sent Event? Instead, it disconnects immediately after returning data.

Question

As the title states, when PHP uses SSE, every time the connection is successful, data is returned, and then the connection is interrupted. Originally, SSE should maintain a long connection, and then return new data to the front end if the data changes. Why can't you maintain a long connection?The browser also enables long connections