Create a folder containing a special character e.g. 'sent & trash'.
Then add a message to that folder.

$client->connect();
$folder = $client->getFolder('sent & trash');
$folder->appendMessage($message); // Not working, showing error

ERROR:
failed to send literal string.
TAG3 APPEND "INBOX.sent & trash" The folder name should be UTF7-IMAP.

Solution:
In the appendMessage () function, replace $this->full_name with $this->path
$this->client->getConnection()->appendMessage($this->path, $message, $options, $internal_date);