we usually set the message at instantiation, like this:
throw new Exception($msg);`
but I am in a situation where I have a default exception object stored in an instance variable and use it through out the objects lifetime, something like this:
throw $this->my_exception;
Since I am reusing the same object, I need to be able to set message at any time before throwning the exception, you see?
Don't do that. It makes tracing the exception harder (since the stacktrace won't include the re-throw). Instead, if you're using 5.3+ use the $previous parameter and make a new exception:
throw new Exception("message", 0, $this->my_exception);
Even if you're using less than 5.3, you can extend the exception class and add it...
Edit: Ok, based on your comments, I see what you're trying to do now. You want to make your class throw a configurable exception. What I would do, is take a string class name in and store that. So $this->my_exception would be a string. You should verify that it's an exception class before storing it since you can't throw something that doesn't extend from Exception:
if (!is_subclass_of($this->my_exception, 'Exception')) {
//Error out, since you can't throw that class name
}
Then, when it's time to throw:
$class = $this->my_exception;
throw new $class("MyMessage");
It's still not great since exceptions are supposed to have semantic meaning (hence the existence of LogicException and InvalidArgumentException), but if it's a requirement, that's not a horrible way of doing it (but pre-instantiating an exception is a horrible way of doing it)...