core-bundle/src/Resources/contao/forms/FormSelectMenu.php
public function parse($arrAttributes=null)
	{
		$strClass = 'select';

		if ($this->multiple)
		{
			$this->strName .= '[]';
			$strClass = 'multiselect';
		}

		// Make sure there are no multiple options in single mode
		elseif (\is_array($this->varValue))
		{
			$this->varValue = $this->varValue[0];
		}

		// Chosen
		if ($this->chosen)
		{
			$strClass .= ' tl_chosen';
		}

		// Custom class
		if ($this->strClass != '')
		{
			$strClass .= ' ' . $this->strClass;
		}

		$this->strClass = $strClass;

		return parent::parse($arrAttributes);
	}
public function parse($arrAttributes=null)
	{
		$strClass = 'select';

		if ($this->multiple)
		{
			$this->strName .= '[]';
			$strClass = 'multiselect';
		}

		// Make sure there are no multiple options in single mode
		elseif (\is_array($this->varValue))
		{
			$this->varValue = $this->varValue[0];
		}

		// Chosen
		if ($this->chosen)
		{
			$strClass .= ' tl_chosen';
		}

		// Custom class
		if ($this->strClass != '')
		{
			$strClass .= ' ' . $this->strClass;
		}

		$this->strClass = $strClass;

		return parent::parse($arrAttributes);
	}
Parse the template file and return it as string

@param array $arrAttributes An optional attributes array

@return string The template markup