(string)$key
Optional. The meta key to retrieve. By default, returns data for all keys. Default empty.
Required: 否
Default: (empty)
(bool)$single
Optional. Whether to return a single value. This parameter has no effect if `$key` is not specified. Default false.
Required: 否
Default: false
返回值
(mixed) An array of values if `$single` is false. The value of meta data field if `$single` is true. False for an invalid `$site_id` (non-numeric, zero, or negative value). An empty string if a valid but non-existing site ID is passed.