core-bundle/src/Resources/contao/library/Contao/Widget.php
protected function isChecked($arrOption)
	{
		if (empty($this->varValue) && empty($_POST) && $arrOption['default'])
		{
			return static::optionChecked(1, 1);
		}

		return static::optionChecked($arrOption['value'], $this->varValue);
	}
protected function isChecked($arrOption)
	{
		if (empty($this->varValue) && empty($_POST) && $arrOption['default'])
		{
			return static::optionChecked(1, 1);
		}

		return static::optionChecked($arrOption['value'], $this->varValue);
	}
Check whether an option is checked

@param array $arrOption The options array

@return string The "checked" attribute or an empty string