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.
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(PHP 4, PHP 5)
ldap_unbind — Unbind from LDAP directory
Unbinds from the LDAP directory.
An LDAP link identifier, returned by ldap_connect().
TRUE on success or
FALSE on failure.