PHP fflush Function

last modified April 3, 2025

The PHP fflush function forces a write of all buffered output to a file pointer. It’s useful when you need immediate output without waiting for the buffer to fill.

Basic Definition

The fflush function flushes the output to a file. It takes one parameter: the file pointer resource. Returns true on success or false on failure.

Syntax: fflush(resource $stream): bool. The function is particularly useful with output streams like STDOUT or file handles.

Basic fflush Example

This shows the simplest usage of fflush with a file handle.

basic_fflush.php

<?php

declare(strict_types=1);

$file = fopen(‘output.txt’, ‘w’); fwrite($file, “Hello, World!\n”); fflush($file); fclose($file);

This writes to a file and immediately flushes the buffer. Without fflush, the data might remain in the buffer until the script ends or the buffer fills.

Flushing STDOUT

fflush can be used with STDOUT for real-time command line output.

stdout_flush.php

<?php

declare(strict_types=1);

for ($i = 0; $i < 5; $i++) { echo “Progress: $i\n”; fflush(STDOUT); sleep(1); }

This demonstrates real-time output in a CLI script. Without fflush, the output might be buffered and appear all at once when the script completes.

Flushing Network Streams

fflush is useful when writing to network sockets or pipes.

socket_flush.php

<?php

declare(strict_types=1);

$socket = fsockopen(’example.com’, 80); fwrite($socket, “GET / HTTP/1.1\r\nHost: example.com\r\n\r\n”); fflush($socket);

while (!feof($socket)) { echo fgets($socket, 128); } fclose($socket);

This flushes the HTTP request immediately to the server. Network programming often requires immediate data transmission rather than buffered output.

Error Handling with fflush

This example shows proper error handling when using fflush.

error_handling.php

<?php

declare(strict_types=1);

$file = fopen(‘data.log’, ‘a’); if ($file === false) { die(“Failed to open file”); }

$bytes = fwrite($file, “Log entry\n”); if ($bytes === false) { die(“Failed to write to file”); }

if (!fflush($file)) { die(“Failed to flush output”); }

fclose($file);

Proper error handling ensures data integrity. Each operation is checked for success before proceeding to the next step in the file handling process.

Performance Considerations

This example demonstrates the performance impact of frequent flushing.

performance.php

<?php

declare(strict_types=1);

$file = fopen(’large_data.txt’, ‘w’);

// Without flushing $start = microtime(true); for ($i = 0; $i < 10000; $i++) { fwrite($file, “Line $i\n”); } $timeNoFlush = microtime(true) - $start;

// With flushing rewind($file); $start = microtime(true); for ($i = 0; $i < 10000; $i++) { fwrite($file, “Line $i\n”); fflush($file); } $timeWithFlush = microtime(true) - $start;

fclose($file);

echo “Without flush: $timeNoFlush seconds\n”; echo “With flush: $timeWithFlush seconds\n”;

Frequent flushing can significantly impact performance. This example compares execution time with and without flushing after each write operation.

Best Practices

• Selective Flushing: Only flush when immediate output is needed.

• Error Handling: Always check the return value of fflush.

• Resource Management: Close files properly after flushing.

• Buffering: Understand PHP’s output buffering mechanisms.

• Performance: Balance between data safety and performance.

Source

PHP fflush Documentation

This tutorial covered the PHP fflush function with practical examples showing its usage in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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