/** * Converts a string to lowercase, returning the result. * * The argument can be any expression as long as it resolves to a string. * * @see http://docs.mongodb.org/manual/reference/operator/aggregation/toLower/ * @see Expr::toLower * @param mixed|Expr $expression * @return self */ public function toLower($expression) { $this->expr->toLower($expression); return $this; }