Denotes that an option has been explicitly set via the script, as a command-line or other parameter, rather than being a default or computed value (that is, the return value of a func).
option_name: String value that represents the built-in or custom option name.
Boolean. Returns 1 (true) if option value has been explicitly set. Returns 0 (false) if option value is a default value, is null, or has not been explicitly set.