Demonstration of output buffering being non-cancellable to capture the full WordPress response

non-cancellable-output-buffer.php

<?php
/**
 * Plugin Name: Non-Cancellable Output Buffer
 * Plugin Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 */

namespace OutputBufferTest;

function process_buffer( $response ) {
	return $response . sprintf( '<!-- Buffer size: %d -->', strlen( $response ) );
}

// Prevent PHP Notice: ob_end_flush(): failed to send buffer.
remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );

// Start output buffering when template starts rendering.
add_action(
	'template_redirect',
	function () {
		ob_start(
			__NAMESPACE__ . '\process_buffer',
			null,
			0 // Neither CLEANABLE, FLUSHABLE, nor REMOVABLE.
		);
	},
	PHP_INT_MIN
);

// This will not have any effect.
add_action( 'wp_footer', 'wp_ob_end_flush_all' );