mysql_close() closes the non-persistent connection to the MySQL server that's associated with the specified link identifier. If link_identifier isn't specified, the last opened link is used.
bool mysql_close ([ resource $link_identifier = NULL ] )
Closes the MySQL Connection.
This extension was deprecated in PHP 5.5.0, and it was removed in PHP 7.0.0.
The MySQLi or PDO_MySQL Should be used.