如果有potential 使用的问题set_object_terms Stephen指出的钩子，可能钩住pre_post_update 然后接着是wp_insert_post就足够了，

add_action(\'pre_post_update\',\'get_terms_before_update\');
function get_terms_before_update(){
    $terms_before = get_the_terms( $id, $taxonomy );
    //do something else if required...
    return $terms_before; 
}

add_action(\'wp_insert_post\',\'get_terms_after_update\');
    function get_terms_after_update(){
    //pass in $terms_before from get_terms_before_update() function
    //compare terms
}

这是Stephen答案的更新，基于新的get\\u terms\\u by function，并假设我们知道需要什么样的分类法。

add_action(\'set_object_terms\',\'save_terms\',10,6);

function save_terms($object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids) {

    //Assuming we only want terms from specific taxonomy
    //If not simply remove this conditional
    $our_taxonomy = \'categories\';
    if($taxonomy == $our_taxonomy){
        //Added terms are specified terms, that did not already exist
        $added_tt_ids = array_diff($tt_ids, $old_tt_ids);

        if ($append) {
            //If appending terms - nothing was removed
            $removed_tt_ids = array();
        } else {
            //Removed terms will be old terms, that were not specified in $tt_ids
            $removed_tt_ids = array_diff($old_tt_ids, $tt_ids);
        }

        /* Note problem 2 by Stephen, we would preferably like the term objects / IDs, 
          Currently we have the taxonomy term IDs */

        foreach($added_tt_ids as $term){
            $added_terms[] = get_term_by( \'term_taxonomy_id\',$term,$taxonomy);
        }

        foreach($removed_tt_ids as $term){
            $removed_terms[] = get_term_by( \'term_taxonomy_id\',$term,$taxonomy);
        }

        //$added_terms contains added term objects
        //$removed_terms contains removed term objects
    }
}