CREATE OR REPLACE FUNCTION FUN_G_P_CTR
(
COST_CTR in varchar(200)
TEMP in varchar(200)
) 
RETURN VARCHAR2 is MAP_PROD varchar(200);

BEGIN
  IF (SUBSTR(COST_CTR,1) = 'S' OR SUBSTR(COST_CTR,1) = 'V' OR SUBSTR(COST_CTR,1) = 'Y') THEN
  -- IF TEMP = 'S' OR TEMP = 'V' OR TEMP = 'Y') THEN
    MAP_PROD := 'PH_BB-RM_SIV';
    ELSE
    MAP_PROD := 'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV'
    END IF;
  RETURN MAP_PROD;
END FUN_G_P_CTR;

SQL> CREATE OR REPLACE FUNCTION FUN_G_P_CTR
  2  (
  3  COST_CTR in varchar(200)
  4  TEMP in varchar(200)
  5  )
  6  RETURN VARCHAR2 is MAP_PROD varchar(200);
  7
  8  BEGIN
  9    IF (SUBSTR(COST_CTR,1) = 'S' OR SUBSTR(COST_CTR,1) = 'V' OR SUBSTR(COST_CTR,1) = 'Y') THEN
 10    -- IF TEMP = 'S' OR TEMP = 'V' OR TEMP = 'Y') THEN
 11      MAP_PROD := 'PH_BB-RM_SIV';
 12      ELSE
 13      MAP_PROD := 'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV'
 14      END IF;
 15    RETURN MAP_PROD;
 16  END FUN_G_P_CTR;
 17  /

Warning: Function created with compilation errors.

SQL> sho err
Errors for FUNCTION FUN_G_P_CTR:
LINE/COL ERROR
-------- -------------------------------------------------------------------------------------
3/20     PLS-00103: Encountered the symbol "(" when expecting one of the following:
         := . ) , @ % default character

CREATE OR REPLACE FUNCTION FUN_G_P_CTR
(
COST_CTR in varchar,
TEMP in varchar
) 
RETURN VARCHAR2 is MAP_PROD varchar(200);

BEGIN
  IF (SUBSTR(COST_CTR,1) = 'S' OR SUBSTR(COST_CTR,1) = 'V' OR SUBSTR(COST_CTR,1) = 'Y') THEN
  -- IF TEMP = 'S' OR TEMP = 'V' OR TEMP = 'Y') THEN
    MAP_PROD := 'PH_BB-RM_SIV';
    ELSE
    MAP_PROD := 'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV';
    END IF;
  RETURN MAP_PROD;
END FUN_G_P_CTR;

select CAST(FUN_G_P_CTR(COST_CTR) AS VARCHAR2(80)) AS "PROD" from STAGING_TABLE1;

Select COST_CTR,
       Case
         When SubStr(COST_CTR,1) In ('S', 'V', 'Y') Then 'PH_BB-RM_SIV'
         Else 'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV'
       End As "PROD"
  From STAGING_TABLE1

Select COST_CTR,
       Case
         When SubStr(COST_CTR,1,1) In ('S', 'V', 'Y') Then 'PH_BB-RM_SIV'
         Else 'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV'
       End As "PROD"
  From STAGING_TABLE1

select cost_ctr,decode(substr(cost_ctr,1,1),'S','PH_BB-RM_SIV',
                                            'V','PH_BB-RM_SIV',
                                            'Y','PH_BB-RM_SIV',
                                            'PH_A'||SUBSTR(COST_CTR,-4)||'_SIDIV') PROD
FROM STAGING_TABLE1;