While it is true that PHP4 allows only one parameter, it is not true that PHP5 allows only 2 parameters. The correct function signature for PHP5 should be:
resource ibase_blob_open([ resource link_identifier, ] string blob_id)
If a link_identifier is not provided, then the "default" link_identifier will be used. The default link identifier is assigned every time you call ibase_connect or ibase_pconnect, so if you have multiple connections it will be whichever was connected LAST.