The payroll information displayed via transaction PA03 is stored within tables T569V, T549A and T549Q. They can also be retrieved using Function module PA03_PCR_READ
Another useful function module could be HR_MX_GET_PAYROLL_PERIOD which retrieves payroll period dates based on the current date, irrespective of what is set in SAP transaction PA03.
CALL FUNCTION 'HR_MX_GET_PAYROLL_PERIOD' EXPORTING PAYROLL_AREA = ld_abkrs * DATE = SY-DATUM IMPORTING * PAYROLL_YEAR = * PAYROLL_PERIOD = PERIOD_BEGIN = ld_start PERIOD_END = ld_end.