WordPress shutdown action vs register_shutdown_function - tweakswp.com tutorial

shutdown-action-vs-register-shutdown.php

<?php
/**
 * WordPress shutdown action vs register_shutdown_function
 *
 * Demonstrates the two mechanisms for running code after WordPress
 * has finished processing. The 'shutdown' action fires within the
 * WordPress lifecycle; register_shutdown_function fires at PHP level
 * after output is flushed.
 *
 * Tutorial: https://tweakswp.com/
 */

// --- Method 1: WordPress 'shutdown' action ---
// Fires after the response is sent to the client (when using fastcgi_finish_request)
// or just before PHP ends execution. Runs while WordPress objects are still in scope.
add_action( 'shutdown', function () {
    // WordPress globals (wpdb, current_user, etc.) are still available here.
    // This is safe for database writes, cache operations, and cleanup tasks.
    error_log( '[shutdown action] WordPress shutdown action fired.' );
} );

// --- Method 2: PHP register_shutdown_function ---
// Called by the PHP engine after the script finishes, including on fatal errors.
// No WordPress context is guaranteed — use only when you need to catch fatal errors.
register_shutdown_function( function () {
    $error = error_get_last();
    if ( $error && in_array( $error['type'], [ E_ERROR, E_PARSE, E_CORE_ERROR, E_COMPILE_ERROR ], true ) ) {
        // Log the fatal error somewhere persistent.
        error_log(
            sprintf(
                '[fatal capture] %s in %s on line %d',
                $error['message'],
                $error['file'],
                $error['line']
            )
        );
    }
} );

// --- Key difference ---
// WordPress 'shutdown' action: add_action( 'shutdown', $callback )
//   - Runs inside WordPress lifecycle
//   - Has access to $wpdb, get_option(), wp_cache_*, WP_User, etc.
//   - Does NOT catch PHP fatal errors (script already dead)
//
// PHP register_shutdown_function( $callback ):
//   - Runs at PHP level after all output is done
//   - Catches E_ERROR (fatal), E_PARSE, E_CORE_ERROR, E_COMPILE_ERROR via error_get_last()
//   - WordPress objects may not be fully available depending on when the fatal occurred