Long-Polling, WebSockets, SSE and Comet: Complete Breakdown with PHP Code

Still refreshing the page manually? Time to go real-time

Imagine: you open an online exchange, and the dollar rate is frozen solid. You hit F5, hit it again — and it doesn't change. Sound familiar? Now imagine the rate updates itself, like in a movie — smoothly and without delays. This isn't magic, it's real-time technologies. They are what turn a static website into a living organism: chats, notifications, dashboards that update in real time.

Today we'll break down the four pillars of the real-time web: Long-Polling, WebSockets, Server-Sent Events (SSE) and Comet. I'll show you how they work "under the hood" and provide ready-to-use PHP code you can implement right now. Let's go!

This article is written for the educational portal pythonlib.ru — where we turn complex concepts into understandable tools.

The gist: what's behind the terms?

When you open a regular page, the browser makes a request, the server responds — and the connection is closed. Simple. But what if the server wants to "knock" on your door with new data on its own? That's where the real fun begins.

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• Long-Polling — the old but reliable grandfather. The client "hangs" on the server and waits for it to deign to respond.
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• WebSockets — the trend of recent years. A full-fledged two-way channel, like a telephone line.
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• Server-Sent Events (SSE) — one-way broadcast from server to client. Ideal for news feeds and stock quotes.
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• Comet — a general term for techniques where the server "pushes" data to the client without an additional request. Long-Polling and SSE are specific cases of Comet.
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Technology #1: Long-Polling — the grandfather of real-time

Long-Polling is when the client tells the server: "I'll wait as long as it takes, just respond when data appears." The client opens a connection and keeps it open until the server sends a response. After receiving the response, the client immediately creates a new request. It's an endless chain.

Pros: works on any hosting, requires no additional libraries, supported by all browsers.

Cons: lots of extra headers, server load, delays with a large number of clients.

PHP Example (Long-Polling)

<?php\n// long_polling.php\nheader('Content-Type: text/plain; charset=utf-8');\nheader('Cache-Control: no-cache');\n\n$timeout = 30; // wait max 30 seconds\n$start = time();\n\n// Simulate new data appearing\n$data = getNewDataFromDB(); // your data retrieval function\n\nwhile (empty($data)) {\n    if ((time() - $start) > $timeout) {\n        echo json_encode(['status' => 'timeout', 'data' => null]);\n        flush();\n        exit;\n    }\n    usleep(500000); // sleep 0.5 seconds\n    $data = getNewDataFromDB();\n}\n\necho json_encode(['status' => 'ok', 'data' => $data]);\nflush();\n?>

On the client side (JavaScript) it will look like this:

function longPoll() {\n    fetch('/long_polling.php')\n        .then(response => response.json())\n        .then(data => {\n            if (data.status === 'ok') {\n                console.log('New data:', data.data);\n                // update the interface\n            }\n            longPoll(); // immediately create a new request\n        })\n        .catch(error => {\n            console.error('Error:', error);\n            setTimeout(longPoll, 1000); // reconnect after one second\n        });\n}\nlongPoll();

See? The client hangs, waits, receives data — and hangs again. This is Long-Polling in PHP in its pure form.

Technology #2: WebSockets — a two-way highway

WebSockets