wp_set_object_terms()

wp_set_object_terms() — Creates term and taxonomy relationships. Relates an object (post, link, etc.) to a term and taxonomy type. Creates the term and taxonomy relationship if it doesn’t already exist. Creates a term if it doesn’t exist (using the slug). A relationship means that the term is grouped in or belongs to the taxonomy. A term has no meaning until it is given context by defining which taxonomy it exists under.

Inputs: $object_id (int): The object to relate to. $terms (string|int|array): A single term slug, single term ID, or array of either term slugs or IDs. Will replace all existing related terms in this taxonomy. Passing an empty array will remove all related terms. $taxonomy (string): The context in which to relate the term to the object. $append (bool): Optional. If false will delete difference of terms. Default false.

Returns array|WP_Error: Term taxonomy IDs of the affected terms or WP_Error on failure.