core-bundle/src/Resources/contao/library/Contao/Widget.php
public static function optionSelected($strOption, $varValues)
	{
		if ($strOption === '')
		{
			return '';
		}

		return (\is_array($varValues) ? \in_array($strOption, $varValues) : $strOption == $varValues) ? ' selected' : '';
	}
public static function optionSelected($strOption, $varValues)
	{
		if ($strOption === '')
		{
			return '';
		}

		return (\is_array($varValues) ? \in_array($strOption, $varValues) : $strOption == $varValues) ? ' selected' : '';
	}
Return a "selected" attribute if the option is selected

@param string $strOption The option to check
@param mixed  $varValues One or more values to check against

@return string The attribute or an empty string