test-object-cache-availability-in-output-buffer-callback-at-shutdown.php

<?php
/**
 * Plugin Name: Test $wp_object_cache availability in output buffer callback in shutdown.
 * Author: Weston Ruter, XWP
 * Description: Add ?erasable=false to the URL to try calling ob_start() with its erasable=false.
 */

if ( ! isset( $_SERVER['REQUEST_URI'] ) || false !== strpos( $_SERVER['REQUEST_URI'], '/wp-admin/' ) ) {
	return;
}

$chunk_size  = 0; // The default value.
$erasable    = ! isset( $_GET['erasable'] ) || (bool) json_decode( $_GET['erasable'] );
$erase_flags = $erasable ? PHP_OUTPUT_HANDLER_STDFLAGS : 0;
ob_start(
	function( $buffer ) use ( $erasable ) {
		$pass   = ! empty( $GLOBALS['wp_object_cache'] );
		$buffer = preg_replace(
			'#(?=</body>)#',
			sprintf(
				'<div style="position:fixed; left: 0; bottom: 0; padding: 1em; color: black; text-shadow: 1px 1px 2px white; z-index: 100000000; background-color: %s">%s; buffer erasable: %s, <a href="?erasable=%s" style="text-decoration: underline; color: blue;">Toggle</a>)</div>',
				$pass ? '#0f0' : '#f00',
				$pass ? '✅ <code>$wp_object_cache</code> exists' : '🚫 <code>$wp_object_cache</code> empty',
				json_encode( $erasable ),
				json_encode( ! $erasable )
			),
			$buffer
		);
		return $buffer;
	},
	$chunk_size,
	$erase_flags
);