term_exists( int|string $term, string $taxonomy = '', int $parent = null )

function term_exists( $term, $taxonomy = '', $parent = null ) {
	global $wpdb;

	$select = "SELECT term_id FROM $wpdb->terms as t WHERE ";
	$tax_select = "SELECT tt.term_id, tt.term_taxonomy_id FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy as tt ON tt.term_id = t.term_id WHERE ";

	if ( is_int($term) ) {
		if ( 0 == $term )
			return 0;
		$where = 't.term_id = %d';
		if ( !empty($taxonomy) )
			return $wpdb->get_row( $wpdb->prepare( $tax_select . $where . " AND tt.taxonomy = %s", $term, $taxonomy ), ARRAY_A );
		else
			return $wpdb->get_var( $wpdb->prepare( $select . $where, $term ) );
	}

	$term = trim( wp_unslash( $term ) );
	$slug = sanitize_title( $term );

	$where = 't.slug = %s';
	$else_where = 't.name = %s';
	$where_fields = array($slug);
	$else_where_fields = array($term);
	$orderby = 'ORDER BY t.term_id ASC';
	$limit = 'LIMIT 1';
	if ( !empty($taxonomy) ) {
		if ( is_numeric( $parent ) ) {
			$parent = (int) $parent;
			$where_fields[] = $parent;
			$else_where_fields[] = $parent;
			$where .= ' AND tt.parent = %d';
			$else_where .= ' AND tt.parent = %d';
		}

		$where_fields[] = $taxonomy;
		$else_where_fields[] = $taxonomy;

		if ( $result = $wpdb->get_row( $wpdb->prepare("SELECT tt.term_id, tt.term_taxonomy_id FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy as tt ON tt.term_id = t.term_id WHERE $where AND tt.taxonomy = %s $orderby $limit", $where_fields), ARRAY_A) )
			return $result;

		return $wpdb->get_row( $wpdb->prepare("SELECT tt.term_id, tt.term_taxonomy_id FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy as tt ON tt.term_id = t.term_id WHERE $else_where AND tt.taxonomy = %s $orderby $limit", $else_where_fields), ARRAY_A);
	}

	if ( $result = $wpdb->get_var( $wpdb->prepare("SELECT term_id FROM $wpdb->terms as t WHERE $where $orderby $limit", $where_fields) ) )
		return $result;

	return $wpdb->get_var( $wpdb->prepare("SELECT term_id FROM $wpdb->terms as t WHERE $else_where $orderby $limit", $else_where_fields) );
}