You must check which version of PHP XAMPP contains, and download appropriate PHP-Ingres package.
As for the DLL you asked for... See, in order to connect to the Ingres database via PHP, you need a minimal installation of ingres (Ingres/NET component is enough for this). So, if you are connecting from Windows, you must download, and install Ingres/NET on that particular Windows machine.
Only Java JDBC driver, and perhaps .NET driver connect straight to the target DAS (Data Access Server).
If you wonder why is this - I suggest you read the documentation and learn about Ingres Architecture. It is not too complex.
In short:
Code:
[Windows machine (Ingres GCN GCC)]
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[Solaris server (Ingres DBMS(es) GCC(s) GCN DMFACP DMFRCP)]
As you can see from above, two Ingres components (GCN - name server, GCC - communication server) are enough to connect to the Ingres installation on the Solaris server. Two GCCs are needed to make that communication possible. (Actually more is going on behind the scene, but this is enough to get the picture). So, on Windows, if you use PHP (and anything else that uses OpenAPI (C library)), you must install Ingres/NET in order to communicate with the Ingres installation on a remote server.