Load a serialized C# object in php?

Short answer: Yes, it is possible.

Middle-sized answer: What you are trying to do is going to teach you a lot about data formats (which is, I think, your aim). It will involve learning about the internal representations (in both languages) of the object and its members as well as the external (serialised) representation. Be prepared for a lot of low-level stuff - what we used to call bit-bashing in days of yore when we practiced it. :)

What you get (apart from useful knowledge) is going to be platform-dependent (machine architecture, compiler/interpreter, supported object types/members, etc) so it's not going to become a general purpose tool. But that's OK - you don't start metalwork by making a swiss army knife.

Best of luck!

Peter
If you think this is a good answer, mark it accepted. Vote anyway.

It would generally be far better to use a custom 'serialisation' process (i.e. making up a string that is easy to parse) than try to de-parse the .Net serialisation format (even the XML one) in another environment. Another responder mentioned JSON; it would also be fairly easy to have your C# code directly produce (and parse, if that is important to your solution) the PHP serialize() format, which is plain text.

Whether you want to actually make a custom serialiser in the .Net sense of the word or just have some methods on your shared class that read/write the appropriate text format is up to you, really.