Class:
CL_HCMFAB_EMPLOYEE_API
Methods:
GET_USERID_FROM_EMPLOYEENUMBER
GET_EMPLOYEENUMBER_FROM_USER

How to use the class:
TRY.
DATA(lv_pernr) = cl_hcmfab_employee_api=>get_instance( )->get_employeenumber_from_user( sy-uname ).
CATCH cx_hcmfab_common.
ENDTRY.