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In PHP, the fflush() function is used to flush the output buffer, forcing the contents of the buffer to be written immediately to the target stream. This function usually works as expected when running in a command-line (CLI) environment, but in a browser environment, its behavior is often disappointing. So why does fflush() seem ineffective in browsers? The answer lies in some special reasons related to PHP and the HTTP protocol.
First, we need to understand PHP’s own output buffering mechanism. In a command-line (CLI) environment, PHP executes code step by step and displays the result immediately. However, in a browser environment, PHP usually stores all output in memory and sends it to the browser only once after the script finishes execution. In other words, in a browser environment, the output is not immediately sent to the client but waits until the script has completed.
Even if you call the fflush() function, PHP will attempt to flush the buffer, but due to browser behavior, data will still only be sent once the script ends. Since the browser receives data based on the HTTP protocol—and HTTP requires the content length to be determined before the response headers are sent—the flush operation does not immediately take effect in the browser.
Interaction between the browser and server follows the HTTP protocol, which defines the basic rules of data transmission. HTTP is based on a request–response model, where a request is sent to the server, and the server processes it and responds to the browser. The response is usually sent only after the server has finished processing the script and prepared all data. Therefore, even if the server uses the fflush() function to flush the buffer, the browser will not receive the content immediately unless the HTTP headers have already been sent.
Since browsers expect to receive the full response headers before processing the body content, any attempt to send data early is constrained by HTTP header requirements. Even if data has been flushed to the buffer via fflush(), it still needs to wait for the response headers to be transmitted and processed.
Browsers typically cache and process data in chunks until they encounter an end-of-transfer signal. The contents of PHP’s output buffer do not directly affect how the browser renders a page. Even if you attempt to flush the buffer with fflush(), the browser will still wait until the server ends the response and sends the complete data before rendering the page. This behavior is why the effect of fflush() is not obvious in a browser environment.
If you want to send partial content immediately in a browser environment, you can use PHP’s ob_flush() function. ob_flush() outputs and clears PHP’s internal output buffer, sending the content immediately to the browser. This differs from fflush(), which works on file streams, while ob_flush() operates on PHP’s output buffer.
For real-time output scenarios—such as live logging or long-running scripts (e.g., data processing or file downloads)—you can combine ob_start() to start output buffering and then use ob_flush() to send content in real time. This ensures partial content reaches the browser earlier instead of waiting until the script finishes entirely.
<span><span><span class="hljs-meta"><?php</span></span><span>
</span><span><span class="hljs-comment">// Start output buffering</span></span><span>
</span><span><span class="hljs-title function_ invoke__">ob_start</span></span><span>();
<p></span>echo "Starting data processing...<br>";<br>
ob_flush();<br>
flush(); // Ensure PHP buffer is sent to the browser</p>
<p>// Simulate a long-running task<br>
sleep(3);</p>
<p>echo "Data processing completed!<br>";<br>
ob_flush(); // Flush and send remaining content<br>
flush();</p>
<p>ob_end_flush(); // End and close the buffer<br>
?><br>
</span>In this example, we first start PHP’s output buffer and then use ob_flush() and flush() to send part of the data to the browser in real time. Even if the script is performing a long-running task, the browser can display “Starting data processing...” while the task runs, and then show “Data processing completed!” after it finishes.
The reason why the fflush() function doesn’t work effectively in a browser environment is mainly due to PHP’s output buffering mechanism and the constraints of the HTTP protocol. The browser won’t immediately receive the server’s output; instead, it waits until the full response is complete. To achieve real-time output in browsers, developers often need to use ob_flush() and flush() to control buffer flushing so that data can be sent earlier. These techniques are especially useful in scenarios such as long-running scripts and real-time data processing.
