<p>Is there a built-in way to know if a given session variable is a serialized object? Say I retrieve a value like $_SESSION['foo'], but I don't know if it was originally a string or if it is a serialized object. Is there some way to check, or once serialized does PHP just see a string as a string as a string? </p>

A string is a string is a string. I think the best you'll be able to do is just try to unserialize it, and if it works, it works. If it doesn't, it doesn't.
The only other option would be to use a regex to see if it "looks" like a serialized object. Just running
```
unserialize()
```
on it might be easier though.