I would like to have a custom back-order message for certain WooCommerce products based on their ID, SKU or something else unique.

Right now I have this, and this works fine for all products that have back-order enabled, but what if I want this logic conditional for only certain product variations?

function backorder_text($availability) {
    if($product->get_sku() == "111100"){
        foreach($availability as $i) {
            //$availability = str_replace('Available on backorder', 'Yo, allow 3-4 weeks for your shiz!', $availability);
            $availability = str_replace('Beschikbaar via nabestelling', 'UITVERKOCHT (U kunt wel bestellen, maar de levertijd is 2-3 werkdagen).', $availability);
        }
        return $availability; 
    }
}
add_filter('woocommerce_get_availability', 'backorder_text');

解决方案

This actually worked best for me:

function backorder_text_prikkabel_zonder_fittingen( $availability, $product ) {
    $product_ids = array( 7631, 7631, 8612, 7629, 7628, 7627 ); 
    if ( in_array( $product->get_id(), $product_ids ) ){ 
        foreach( $availability as $i ) {
            $availability = str_replace( 'Default back-order message', 'Your custom back-order message', $availability );
        }
    }
    return $availability;
}
add_filter('woocommerce_get_availability', 'backorder_text_prikkabel_zonder_fittingen', 10, 2 );

Explanation: the array contains a list of product variation ID's. If you echo $product->get_id(), somewhere around the bottom of the page, you will see a list of ID's for each of the products variations for that product.

Make sure the default order message exactly matches the message you get on your product page so the custom message will be used instead.