This is not so much a feature of Orion as it is a feature of ASP.NET.
If you are looking simply to embed PHP into the Orion ASP.NET resources (ASPX and ASCX pages) then there are tools out there to facilitate this. SolarWinds does not endorse any of these products, but I have read about Phalanger and other tools that allow you to run PHP inside of ASP.NET pages.
Once you start changing your code in Orion, we can't necessarily support your customizations, so you are on your own as far as that goes, but I have read that many people have had great success blending their ASP.NET and PHP applications outside of Orion, so I don't see why you wouldn't be able to do this inside of Orion.
Similarly, anyone should be able to change the C# code in the plain text Orion pages for their own benefit, at the same support cost.