I tried to get the backup function working, but could never figur out where to get the resource service_handle. The trick was to use ibase_service_attach.
$resource_service_handle = ibase_service_attach ('localhost', 'username', 'password');
ibase_backup($resource_service_handle, 'source_database', 'destination_file');
ibase_service_detach ($resource_service_handle);
ibase_affected_rows