Hi,
When $mailbox string ends with '/' it doesn't subscribe and doesn't return any error. You need to rtrim this character.(PHP 4, PHP 5, PHP 7, PHP 8)
imap_subscribe — Suscribe a un buzón de correo
Suscribe a un buzón de correo.
imapAn IMAP\Connection instance.
mailboxEl nombre del buzón de correo, ver la documentación de la función imap_open() para más detalles
Passing untrusted data to this parameter is insecure, unless imap.enable_insecure_rsh is disabled.
| Versión | Descripción | 
|---|---|
| 8.1.0 | The imapparameter expects an IMAP\Connection
  instance now; previously, a validimapresource was expected. | 
