Automatically add a tag according to custom metadata

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I accidentally found this question searching for something similar, and I liked your approach, so I improved your code a bit.

  1. I updated the action hook so it’s being triggered on save AND update.
  2. Added the wp_remove_object_terms to be able to switch back ( toggle ) custom meta box value ( in this case check-box value ).

function works fine with both, regular and custom post types, also can be used with tags, categories or custom taxonomies too.

function set_term( $post_id, $your_term ){

    $post_id = get_the_ID();

    $your_term = get_post_meta( $post_id, 'your_custom_meta_id', true ); 

    // check the custom meta-box checkbox value
    if ( $your_term == '1' ) {
        // Create a new term if checked
        wp_set_object_terms( $post_id, 'YourTerm', 'your_custom_taxonomy', true );
    } else {
        // Remove the created term if unchecked
        wp_remove_object_terms( $post_id, 'YourTerm', 'your_custom_taxonomy' );
    }
}

add_action( 'save_post', 'set_term', 10, 3 );