ldap_unbind kills the link descriptor.  So, if you want to rebind as another user, just bind again; don't unbind.  Otherwise, you'll have to open up a new connection.(PHP 4, PHP 5, PHP 7, PHP 8)
ldap_unbind — Unbind from LDAP directory
| Version | Description | 
|---|---|
| 8.1.0 | The ldapparameter expects an LDAP\Connection
  instance now; previously, a validldap linkresource was expected. | 
ldap_unbind kills the link descriptor.  So, if you want to rebind as another user, just bind again; don't unbind.  Otherwise, you'll have to open up a new connection.