code: protected function getReturnType($method)
      {
          // Guard: method getReturnType() is only supported by PHP7.0+
          if (PHP_VERSION_ID >= 70000) {
              $returnType = (string) $method->getReturnType();
  
              if('self' === $returnType) {
                  $returnType = "\\".$this->method->getDeclaringClass()->getName();
              }
  
              // Guard: method getReturnType()->allowsNull() is only supported by PHP7.1+
              if(PHP_VERSION_ID >= 70100) {
                  $returnType = '?'.$returnType;
              }
  
              if($returnType != '') {
                  return ': '. $returnType;
              }
          } 
              
          // the return type feature doesn't exist below PHP7, return empty string
          return '';        
      }
docs: getReturnType
  
  @param ReflectionMethod $method   Method object from reflection API
  
  @return string The Parameters string for a method.