/** * Returns a substring of a string, starting at a specified index position * and including the specified number of characters. The index is zero-based. * * The arguments can be any valid expression as long as long as the first argument resolves to a string, and the second and third arguments resolve to integers. * * @see http://docs.mongodb.org/manual/reference/operator/aggregation/substr/ * @see Expr::substr * @param mixed|Expr $string * @param mixed|Expr $start * @param mixed|Expr $length * @return self */ public function substr($string, $start, $length) { $this->expr->substr($string, $start, $length); return $this; }