A previous comment noted: "I've also noticed that the departmentNumber, employeeNumber (and maybe others in inetorgperson.schema) are not returned from a search."
This is incorrect. These attributes are returned, but you must reference them with lowercase names. That is, instead of doing this:
$entries[0]["departmentNumber"][0]
Do this:
$entries[0]["departmentnumber"][0]
This doesn't seem like "correct" behavior to me, but I don't know enough about LDAP to say for sure.