core-bundle/src/Resources/contao/library/Contao/Widget.php
public function __isset($strKey)
	{
		switch ($strKey)
		{
			case 'id':
				return isset($this->strId);
				break;

			case 'name':
				return isset($this->strName);
				break;

			case 'label':
				return isset($this->strLabel);
				break;

			case 'value':
				return isset($this->varValue);
				break;

			case 'class':
				return isset($this->strClass);
				break;

			case 'template':
				return isset($this->strTemplate);
				break;

			case 'wizard':
				return isset($this->strWizard);
				break;

			case 'required':
				return isset($this->arrConfiguration[$strKey]);
				break;

			case 'forAttribute':
				return isset($this->blnForAttribute);
				break;

			case 'dataContainer':
				return isset($this->objDca);
				break;

			case 'activeRecord':
				return isset($this->objDca->activeRecord);
				break;

			default:
				return isset($this->arrAttributes[$strKey]) || isset($this->arrConfiguration[$strKey]);
				break;
		}
	}
public function __isset($strKey)
	{
		switch ($strKey)
		{
			case 'id':
				return isset($this->strId);
				break;

			case 'name':
				return isset($this->strName);
				break;

			case 'label':
				return isset($this->strLabel);
				break;

			case 'value':
				return isset($this->varValue);
				break;

			case 'class':
				return isset($this->strClass);
				break;

			case 'template':
				return isset($this->strTemplate);
				break;

			case 'wizard':
				return isset($this->strWizard);
				break;

			case 'required':
				return isset($this->arrConfiguration[$strKey]);
				break;

			case 'forAttribute':
				return isset($this->blnForAttribute);
				break;

			case 'dataContainer':
				return isset($this->objDca);
				break;

			case 'activeRecord':
				return isset($this->objDca->activeRecord);
				break;

			default:
				return isset($this->arrAttributes[$strKey]) || isset($this->arrConfiguration[$strKey]);
				break;
		}
	}
Check whether an object property exists

@param string $strKey The property name

@return boolean True if the property exists