ob_start() / ob_get_clean() Since: PHP 4（2000）

// Starts output buffering.
ob_start($callback, $chunk_size, $flags);

// Returns the contents of the buffer. The buffer is kept intact.
ob_get_contents();

// Returns the contents of the buffer and ends buffering.
ob_get_clean();

// Outputs the contents of the buffer and ends buffering.
ob_end_flush();

<?php
// Basic usage of output buffering.
ob_start();
echo "This output is stored in the buffer.";
echo "It has not been displayed on screen yet.";
$content = ob_get_clean(); // Retrieves the buffer contents and ends buffering.
echo mb_strlen($content, "UTF-8") . " characters of output were captured.";

// Capturing var_dump() output as a string.
ob_start();
var_dump(['PHP', 'JavaScript', 'Python']);
$debug = ob_get_clean();
error_log($debug); // Writes the debug info to the log.

// Using output buffering to generate HTML templates.
function render_card($title, $body) {
	ob_start();
	echo '
';
	echo '
' . htmlspecialchars($title) . '
';
	echo '
' . htmlspecialchars($body) . '
';
	echo '
';
	return ob_get_clean();
}

$html = render_card("Notice", "The site has been relaunched.");
echo $html;

// Suppressing output before sending a header() redirect.
ob_start();
echo "This output will be discarded before the redirect.";
ob_end_clean(); // Discards the buffer.
header("Location: /login.php");
exit;

// Using a callback to transform output.
ob_start(function ($buffer) {
	return mb_strtoupper($buffer); // Converts all output to uppercase.
});
echo "hello world";
ob_end_flush(); // Outputs 'HELLO WORLD'.

ob_start() begins output buffering, storing all subsequent output from echo, print_r(), and similar functions into an in-memory buffer. The buffered output can then be retrieved as a string with ob_get_clean(), or sent to the browser all at once with ob_end_flush().

Common uses for output buffering include capturing var_dump() output as a string, rendering partial HTML templates, and suppressing output before a header() call. In PHP, any output sent before header() or setcookie() causes an error — output buffering lets you avoid this problem.

Buffering can be nested, and you can check the current nesting depth with ob_get_level(). For debug output, see var_dump(). For basic output, see echo / print.