code: public function setNode($strName, $varValue = null)
      {
          if (!isset($this->children()->$strName)) {
              if ($varValue === null) {
                  return $this->addChild($strName);
              } else {
                  return $this->addChild($strName, $varValue);
              }
          } else {
              if ($varValue === null) {
                  unset($this->children()->$strName);
                  return null;
              } else {
                  $this->children()->$strName = (string)$varValue;
                  return $this->children()->$strName;
              }
          }
      }
docs: Set (or create) Node
  
  @param string $strName
  @param string $varValue
  @return null|\static|\static[]