Hello,
As far as QuickBooks goes, a PO is a NON-posting transaction. No money is changing hands and no accounts are being affected.
If you didn't receive the items OR bill until 2019, then just enter the bill and receive the items in QuickBooks with the 2019 date same as you normally would. This will show that your inventory asset for the items increased in 2019 as well as your accounts payable increased in 2019 for the bill.