Fatal error: Exception thrown without a stack frame in Unknown on line 0

While Christer were updating his profile on one of our current projects, he suddenly got this rather cryptic message. We tossed a few ideas around, before just leaving it for the day. We came back to the issue earlier today, and I suddenly had the realization that it had to have something to do with the session handling. Changing our session handler from memcache to the regular file based cache did nothing, so then it had to be something in the code itself.

The only thing we have in our session is an object representing the currently logged in user, so it had to be something in regards to that. After a bit of debugging I found the culprit; a reference to an object which contained a reference to a PDO object. PDO objects cannot be serialized, and this exception was being thrown after the script had finished running. The session is committed to the session storage handler when the script terminates, and therefor is run out of the regular script context (and this is why you get “in Unknown on line 0”. It would be very helpful if the PHP engine had been able to provide at least the message from the exception, but that’s how it currently is.

Hopefully someone else will get an Eureka!-moment when reading this!

The solution we ended up with was to remove the references to the objects containing the unserializable structures. This was done by implementing the magic __sleep method, which returns a list of all the fields that should be kept when serializing the object (we miss the option of instead just unsetting the fields that needs to be unset and then let the object be serialized as it would have if the __sleep method wasn’t implemented). We solved this by implemeting the __sleep method and removing our references, before returning all the fields contained in the object:

public function __sleep()
    $this->manager = null;
    return array_keys(get_object_vars($this));

And there you have it, one solved problem!