I would say your best bet is to write some sort of standards compliant wrapper around your cache API. Something like a REST service that works equally well in PHP and ASP.NET.
So in that solution you would have :
- ASP.NET Web Application
- ASP.NET Hosted Web API -or- ASP.NET Hosted WCF webHttpBinding
- PHP Hosted Web Application
Your two web applications should then share serialized JSON or XML objects through the REST service.