Output Buffering (ob_start / ob_flush)

Also Known As

TL;DR

Explanation

PHP's output buffering (ob_start(), ob_get_clean(), ob_flush()) intercepts all echo/print output into an internal buffer. This enables: modifying output after generation (adding compression, injecting content), sending headers after output has started (workaround for 'headers already sent' errors — though proper code shouldn't need this), capturing template output as a string, and implementing full-page caching. Nested buffers are supported. ob_gzhandler compresses output — but prefer server-level compression. Forgetting to flush or clean a buffer on exceptions is a common source of partially-delivered or corrupted responses.

Common Misconception

Why It Matters

Common Mistakes

• Calling header() after output has been sent — 'headers already sent' error caused by missing ob_start().
• Not calling ob_end_clean() or ob_end_flush() — nested buffers cause memory leaks in long-running scripts.
• Relying on output buffering to fix poorly structured code instead of fixing the output order.
• Not knowing that PHP-FPM and some SAPIs have implicit output buffering — behaviour differs across environments.

Code Examples

// Headers already sent — ob_start() would fix this:
echo 'Some content'; // Output sent
header('Location: /dashboard'); // Warning: headers already sent

// Fix:
ob_start();
echo 'Some content';
header('Location: /dashboard');
ob_end_clean();
exit;

// ob_start() captures all output until ob_end_flush()/ob_get_clean()
ob_start();
require 'template.php'; // outputs HTML
$html = ob_get_clean(); // capture and stop buffering

// Send custom headers before any output — ob_start() buys time
ob_start();
// ... lots of processing that might echo ...
header('X-Processing-Time: ' . $elapsed); // still works — nothing sent yet
ob_end_flush();

// Template rendering pattern
function render(string $template, array $vars): string {
    extract($vars, EXTR_SKIP);
    ob_start();
    require "views/{$template}.php";
    return ob_get_clean();
}
$html = render('email/welcome', ['user' => $user]);

References

• ↗ https://www.php.net/manual/en/book.outcontrol.php

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