ldap_bind(): Unable to bind to server: Protocol error

Note that you have to specify the protocol version prior to making a call to ldap_bind, when the server is expecting LDAP protocol version 3. If you do not, you will receive a warning and fail to bind, such as:

ldap_bind(): Unable to bind to server: Protocol error

In order to avoid this, make this call:

ldap_set_option($ds, LDAP_OPT_PROTOCOL_VERSION, 3);
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Where $ds is the result returned by ldap_connect(...);