/** * Converts a string to uppercase, 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/toUpper/ * @see Expr::toUpper * @param mixed|Expr $expression * @return self */ public function toUpper($expression) { $this->expr->toUpper($expression); return $this; }