库存变化事件、触发电子邮件和添加订单备注后的可用挂钩位于
https://github.com/woocommerce/woocommerce/blob/master/includes/wc-stock-functions.php
在这里我们找到函数wc_trigger_stock_change_notifications()，它包含一些动作钩子
可用挂钩：

// No stock
function action_woocommerce_no_stock( $wc_get_product ) {
    // make action magic happen here... 
}
add_action( 'woocommerce_no_stock', 'action_woocommerce_no_stock', 10, 1 );

// Low stock
function action_woocommerce_low_stock( $wc_get_product ) { 
    // make action magic happen here... 
}
add_action( 'woocommerce_low_stock', 'action_woocommerce_low_stock', 10, 1 );

// On backorder
function action_woocommerce_product_on_backorder( $array ) { 
    // make action magic happen here... 
}
add_action( 'woocommerce_product_on_backorder', 'action_woocommerce_product_on_backorder', 10, 1 );

因此，您可以使用woocommerce_low_stock和CRUD Objects in 3.0

function action_woocommerce_low_stock( $wc_get_product ) {    
    // Product set tag id(s), multiple IDs can be added, separated by a comma
    $new_tag_ids = array( 'YOUR TAG ID', 'ANOTHER TAG ID' );
    $wc_get_product->set_tag_ids( $new_tag_ids );

    // OPTIONAL: Set category ids
    //$wc_get_product->set_category_ids( array( 39, 2 ) );

    // Save
    $wc_get_product->save();
}
add_action( 'woocommerce_low_stock', 'action_woocommerce_low_stock', 10, 1 );

**注意）**如果您不只是想为产品添加新的标签，还想保留现有的标签。
替换：

// Product set tag id(s), multiple IDs can be added, separated by a comma
$new_tag_ids = array( 'YOUR TAG ID', 'ANOTHER TAG ID' );
$wc_get_product->set_tag_ids( $new_tag_ids );

带：

// Get current tag id(s)
$current_tag_ids = $wc_get_product->get_tag_ids();

// Product set tag id(s), multiple IDs can be added, separated by a comma
$new_tag_ids = array( 'YOUR TAG ID', 'ANOTHER TAG ID' );
$wc_get_product->set_tag_ids( array_unique( array_merge( $current_tag_ids, $new_tag_ids ), SORT_REGULAR ) );