Following goetz at rvs dot uni-hannover dot de's comment, it is not exactly true that ldap_modify can't change objectclass.
In fact, it's impossible to add an objectclass which is set as STRUCTURAL in the schema description. But you can add an objectclass set as AUXILIARY. See OpenLDAP FAQ below for explanation why :
http://www.openldap.org/faq/data/cache/1341.html
For example, if you have an ldap entry of type "device", which is a structural objectclass, named "cn=myNetCard,ou=Networks,dc=example,dc=com", you can do this :
<?php
$entry["objectclass"][0] = "device";
$entry["objectclass"][1] = "ieee802Device"; // add an auxiliary objectclass
$entry["macAddress"][0] = "aa:bb:cc:dd:ee:ff";
ldap_modify ($cnx, "cn=myNetCard,ou=Networks,dc=example,dc=com", $entry);
?>
But this will not work :
<?php
$entry["objectclass"][0] = "device";
$entry["objectclass"][1] = "ipNetwork"; // add a structural objectclass
$entry["ipNetworkNumber"][0] = "1.2.3.4";
ldap_modify ($cnx, "cn=myNetCard,ou=Networks,dc=example,dc=com", $entry);
?>
You will get the following error : "Cannot modify object class"