Problem with url import to acf image field

i figured out that i have to use the image id to update the acf-image field. i tried this customization but without success:

function really_simple_csv_importer_save_meta_filter( $meta, $post, $is_update ) {
$attachment_url = $meta[‘field_55f2be2a39177’];
$image_id = pn_get_attachment_id_from_url( $attachment_url );
// function to retrieve the image id that i need for the update_field()
$field_key = “field_55f2be2a39177”;
update_field($field_key, $image_id);
return $meta;}
add_filter( ‘really_simple_csv_importer_save_meta’, ‘really_simple_csv_importer_save_meta_filter’, 10, 3 );