Metadata (category)

add_metadata()	Add metadata (an additional data) for the specified object (a post, a comment, a user etc).
delete_metadata()	Deletes the metadata of the specified object (post, user, comment).
delete_metadata_by_mid()	Deletes a row of metadata from the Database by the primary field `meta_id`.
get_metadata()	Gets the metadata of the specified object (post, comment, user). Caches the result.
is_protected_meta()	Determines whether a meta key is protected (internal).
register_meta()	Registers a meta key (key of custom field).
sanitize_meta()	Clears the value of meta data. The function itself does nothing, but applies the filter `"sanitize_{$meta_type}_meta_{$meta_key}"`, through which different meta data can be sanitized differently.
update_metadata()	Update metadata for the specified object. If no value already exists, the metadata will be added as new metadata.