Returns an error number for the server (last request) identified by id.
yaz_errno should be called after network activity for each server - (after yaz_wait returns) to determine the success or failure of the last operation (e.g. search).
The connection resource returned by yaz_connect.
Returns an error code. The error code is either a Z39.50 diagnostic code (usually a Bib-1 diagnostic) or a client side error code which is generated by PHP/YAZ itself, such as "Connect failed", "Init Rejected", etc.