PHP Fatal error: Uncaught Error: Undefined constant "monthly_orders" – in functions.php WordPress php 8 update

I am updating Php version of wordpress website to php 8.0. I am getting fatal errors of undefined variables.

PHP Fatal error: Uncaught Error: Undefined constant "monthly_orders" in /www/…./public/wp-content/themes/…../functions.php:1263

Here is the part of code.

function wc_new_monthly_orders_column($columns)
{
    
    $new_columns = array();
    foreach ($columns as $column_name => $column_info) {
        $new_columns[$column_name] = $column_info;
        if ('orders' === $column_name) {
        $new_columns[monthly_orders] = __('Monthly Orders', 'theme_domain');
        }
    }
return $new_columns;
}
add_filter('manage_edit-shop_subscription_columns', 'wc_new_monthly_orders_column');

I am not really sure how to fix this.

>Solution :

I think you need to use double quotes like $new_columns[monthly_orders] and if monthly_orders is a variable then use $new_columns[$monthly_orders]