Sap Script- FM to get WBS element (PS_POSID) by using Int. Num.(PS_PSP_PNR)

Hi Friends,

I am working on PO sapscript. I got value ekkn-ps_psp_pnr(Internal Number) by using ekko,ekpo into my subroutine. But now I need to print cobl-ps_posid (WBS element) value.

I heard that the both fields have inter-related. Please anybody give me the Function Module name which will use to get ps_posid value by using ps_psp_pnr.