Log WooCommerce status changes with stack trace, even when WP_DEBUG is off.

squarecandy-wc-status-change-logger.php

<?php
/**
 * Plugin Name: SquareCandy Status Change Logger for WooCommmerce
 * Description: Log WooCommerce status changes with stack trace, even when WP_DEBUG is off.
 * Author:      squarecandy
 * License:     GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */
 
add_action( 'woocommerce_order_status_changed', 'squarecandy_status_change_logger', 999, 4 );
function squarecandy_status_change_logger( $order_id, $old_status, $new_status, $order ) {

	// just log orders that move to cancelled, but also all REST API requests
	// if ( 'cancelled' !== $new_status && ! defined( 'REST_REQUEST' ) ) {
	//	return;
	// }

	$log =  '===============' . PHP_EOL;
	$log .= 'Order #' . $order_id . ' status changed from ' . $old_status . ' to ' . $new_status . PHP_EOL;
	$log .= '----------' . PHP_EOL;
	// $log .= print_r( $order, true ) . PHP_EOL;
	// $log .= '----------' . PHP_EOL;
	$log .= 'Debug Backtrace' . PHP_EOL;
	$log .= '----------' . PHP_EOL;
	foreach ( debug_backtrace() as $i => $item ) {
		if ( 0 === $i ) {
			$log .= $item['args'][1] . PHP_EOL;
		} else {
			$log .= $item['file'] . ' -- line ' . $item['line'] . PHP_EOL;
		}
	}
	if ( defined( 'REST_REQUEST' ) ) :
		$log .= '---------------' . PHP_EOL;
		$log .= 'THIS IS A REST API REQUEST' . PHP_EOL;
		$log .= '-------HEADERS--------' . PHP_EOL;
		$log .= print_r( getallheaders(), true );
		$log .= '-------POST--------' . PHP_EOL;
		$log .= print_r( $_POST, true );
		$log .= '-------GET--------' . PHP_EOL;
		$log .= print_r( $_GET, true );
		$log .= '--------ORDER-------' . PHP_EOL;
		$log .= print_r( $order, true ) . PHP_EOL;
	endif;
	$log .= '===============' . PHP_EOL;
	$file = wp_get_upload_dir()['basedir'] . '/order_status_change.log';
	file_put_contents($file, $log, FILE_APPEND | LOCK_EX);
}