code: public function getSlavePdo($fallbackToMaster = true)
      {
          $db = $this->getSlave(false);
          if ($db === null) {
              return $fallbackToMaster ? $this->getMasterPdo() : null;
          }
  
          return $db->pdo;
      }
docs: Returns the PDO instance for the currently active slave connection.
  When [[enableSlaves]] is true, one of the slaves will be used for read queries, and its PDO instance
  will be returned by this method.
  @param bool $fallbackToMaster whether to return a master PDO in case none of the slave connections is available.
  @return PDO the PDO instance for the currently active slave connection. `null` is returned if no slave connection
  is available and `$fallbackToMaster` is false.