My misunderstanding of OpenLDAP API
I used to think that ldap_unbind() is the opposite of ldap_bind(), however I was wrong. Pierangelo Masarati (ando at openldap) pointed out that it was actually opposite of ldap_init(). So, do NOT expect init() -> bind() -> search() -> unbind() -> bind() would success, it’s simply wrong thing. You do not need to unbind() before you do another bind().
Just make a note.