(PHP >= 5.3.0, PECL phar >= 2.0.0)
PharData::isWritable — Returns true if the tar/zip archive can be modified
   This method returns TRUE if the tar/zip archive on disk is not read-only.
   Unlike Phar::isWritable(), data-only tar/zip archives
   can be modified even if phar.readonly is set to 1.
  
No parameters.
   Returns TRUE if the tar/zip archive can be modified