Summary: These functions get or set site option variables according to user specification.
Detail: The full function names with their variables and brief descriptions are as follows:
get_site_option( $key, $default = false, $use_cache = true )
This function checks for the existence of the option named $key. By default, the function will first in the wp_cache and failing that, in the database. Should the key exist in the database but not the cache, it will be added. If it appears in neither location, the $default value “false” will be added to the cache. By setting $use_default to false, you can override the option to use the cache in the query, but the cache is still updated with the returned value.
add_site_option( $key, $value )
This function first checks for the existence of the option $key in the site meta table, and if that option exists, calls update_site_option(). Otherwise, it first serializes objects and arrays passed as $value, then creates and sets the option. Additionally, this function will destroy any wp-cache’d reference to the option.
update_site_option( $key, $value )
This function is essentially the same as add_site_option in most respects, except that it assumes the option exists and only calls add_site_option if updating the option returns false. Technically, this would be the more efficient function to call unless it is certain that the option does not exist.