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//hook into the init action and call create_book_taxonomies when it fires
add_action( 'init', 'create_subjects_hierarchical_taxonomy', 0 );
//create a custom taxonomy name it subjects for your posts
function create_subjects_hierarchical_taxonomy() {
// Add new taxonomy, make it hierarchical like categories
//first do the translations part for GUI
$labels = array(
'name' => _x( 'Subjects', 'taxonomy general name' ),
'singular_name' => _x( 'Subject', 'taxonomy singular name' ),
'search_items' => __( 'Search Subjects' ),
'all_items' => __( 'All Subjects' ),
'parent_item' => __( 'Parent Subject' ),
'parent_item_colon' => __( 'Parent Subject:' ),
'edit_item' => __( 'Edit Subject' ),
'update_item' => __( 'Update Subject' ),
'add_new_item' => __( 'Add New Subject' ),
'new_item_name' => __( 'New Subject Name' ),
'menu_name' => __( 'Subjects' ),
);
// Now register the taxonomy
register_taxonomy('subjects',array('books'), array(
'hierarchical' => true,
'labels' => $labels,
'show_ui' => true,
'show_in_rest' => true,
'show_admin_column' => true,
'query_var' => true,
'rewrite' => array( 'slug' => 'subject' ),
));
}