Some of the code is likely to break under PHP5:
It is in the Send method of Connection.php
return new Net_HL7_Response($data);
$ret = new Net_HL7_Response($data);
You may even want to add a return by reference for the method naming it &send instead of send.
I find the classes layout a bit weird, the base file containing almost nothing but as I don't dont know HL7 I won't make suggestions which could prove off target later :)