try this one:
CREATE OR REPLACE FUNCTION prva_xml(STR_XML IN VARCHAR2 default null
,STR_XSD IN VARCHAR2 default null) RETURN BOOLEAN IS
XMLSCHEMA XMLTYPE;
RES BOOLEAN;
BEGIN
XMLSCHEMA := XMLTYPE('
ESEMPIO
xdb:SQLType="PURCHASEORDER_T"
');
IF (DBMS_XDB.EXISTSRESOURCE('/public/ticket.xsd')) THEN
DBMS_XDB.DELETERESOURCE('/public/ticket.xsd');
END IF;
RES := DBMS_XDB.CREATERESOURCE('/public/ticket.xsd', XMLSCHEMA);
COMMIT;
DBMS_XMLSCHEMA.REGISTERSCHEMA(SCHEMAURL => 'ticket20.xsd', SCHEMADOC => XDBURITYPE('/public/ticket.xsd').GETCLOB(), LOCAL => FALSE, GENTYPES => TRUE, GENBEAN => FALSE, GENTABLES => TRUE, OWNER => USER);
RETURN TRUE;
end prva_xml;