code: private static function toVariableName(string $name): string
      {
          $variableName = preg_replace('/[^A-Za-z0-9]/', '', $name);
          if ($variableName{0} >= '0' && $variableName{0} <= '9') {
              $variableName = 'a'.$variableName;
          }
  
          return '$'.$variableName;
      }
docs: Transform $name into a valid variable name by removing not authorized characters or adding new ones.
  
  - foo => $foo
  - $foo => $foo
  - fo$o => $foo
  
  @param string $name
  
  @return string