9.1 The payment application must be developed such that the database server and web server are not required to be on the same server, nor is the database server required to be in the DMZ with the web server.
PCI Data Security Standard Requirements 1.3 and 1.3.4
Testing Procedures:
9.1.a To verify that the payment application stores cardholder data in the internal network, and never in the DMZ, obtain evidence that the payment application does not require data storage in the DMZ, and will allow use of a DMZ to separate the Internet from systems storing cardholder data (e.g., payment application must not require that the database server and web server be on the same server, or in the DMZ with the web server).
9.1.b If customers could store cardholder data on a server connected to the Internet, examine PA-DSS Implementation Guide prepared by vendor to verify customers and resellers/integrators are told not to store cardholder data on Internet-accessible systems (e.g., web server and database server must not be on same server).