First of all connect as usual. (Some servers require authentication to get the 
RootDSE entry)

$config = array( 'host' => 'localhost' );
$ldap = Net_LDAP2::connect( $config );
if( Net_LDAP2::isError( $ldap ) ) die( $ldap->getMessage() );

Now we can get the entry:

$dse = $ldap->rootDSE();
if( Net_LDAP2::isError( $dse ) die( $dse->getMessage() );

You can give an array of attributes to fetch  as an parameter ro rootDSE().
If none are given these ones are fetched:

namingContexts
altServer
supportedExtension
supportedControl
supportedSASLMechanisms
supportedLDAPVersion
subschemaSubentry

Then you can work with the object:

$basedn = $dse->getValue( 'namingContexts' );

if( $dse->supportedVersion( 3 ) == 3 ) {
  do_something_only_ldap_v3_can_do();
}

Public functions:

getValue( string )

get the value of this attribute. same syntax as Net_LDAP2_Entry::get_value()

supportedControl( oid ) 
supportedExtension( oid )

check if the given control/extension is supported by the server

supportedSASLMechanism( mechanism )

check if the given sasl mechanism is supported by the server

supportedVersion( version )

check if the given ldap version is supported by the serve

These are alias functions of the above, to make the api perl-ldap compatible.

get_value()
supported_control()
supported_extension()
supported_sasl_mechanism()
supported_version()