SCOTT@orcl_11gR2> CREATE OR REPLACE FUNCTION test_func
  2    (p_ColumnAmt IN NUMBER,
  3  	p_ColumnNm  IN VARCHAR2)
  4    RETURN	       NUMBER
  5  AS
  6    v_RnkNoAmt      NUMBER;
  7  BEGIN
  8    SELECT EXTRACTVALUE
  9  		(XMLTYPE
 10  		   (DBMS_XMLGEN.GETXML
 11  		      ('SELECT RANK (' || p_ColumnAmt || ')
 12  				 WITHIN GROUP (ORDER BY ' || p_ColumnNm || ' DESC) rank
 13  			FROM   emp')),
 14  		 '/ROWSET/ROW/RANK')
 15    INTO   v_RnkNoAmt
 16    FROM   DUAL;
 17    RETURN v_RnkNoAmt;
 18  END test_func;
 19  /

Function created.

SCOTT@orcl_11gR2> SHOW ERRORS
No errors.
SCOTT@orcl_11gR2> SELECT test_func (20, 'DEPTNO') FROM DUAL
  2  /

TEST_FUNC(20,'DEPTNO')
----------------------
                     8

1 row selected.

SCOTT@orcl_11gR2> SELECT test_func (7900, 'EMPNO') FROM DUAL
  2  /

TEST_FUNC(7900,'EMPNO')
-----------------------
                      3

1 row selected.

SCOTT@orcl_11gR2>