Error in custom function returning a currency symbol

Hi all, I have a custom function to convert money value. If I add the currency symbol to the return value I receive this error:

ParseError: syntax error, unexpected '$', expecting ',' or ';'
path_to_index.volt.compiled (78)

I've discovered that the error is generated by the final symbol (if I remove it, the function and the template work). Is it a volt's bug? Is there a way to return a value with the currency symbol?
This is the simple piece of code

$volt = $this->di->get('voltService');
$compiler = $volt->getCompiler();
$compiler->addFunction('currency_conversion', function($resolvedArgs, $exprArgs) {
  $var = \App\Models\Currency::conversion($exprArgs[0]['expr']['value'],$exprArgs[1]['expr']['value']);
  return $var;
});

UPDATE: It throws a syntax error even if I use "EUR" or "USD" instead of the symbol. The convert function is:

 public static function conversion($value,$currency=null){
      $di = \Phalcon\DI::getDefault();
      if(isset($currency)) {
           $cur=self::findFirstByCode($currency);
           $exchange = $value * $cur->getValue();
      } elseif( $di->get('session')->get('currency') ) {
           $currency = $di->get('session')->get('currency');
           $cur=self::findFirstByCode($currency);
           $exchange = $value * $cur->getValue();
      } else {
           $currency='EUR';
           $cur=self::findFirstByCode($currency);
           $exchange = $value * $cur->getValue();
      }
      return (string)( round($exchange,2).' '.$cur->getCode() );
 }