serialized class instance disappears from session - php

Question

I've made an img editor which works fine in 99.9% of the situations.
However, in that rare 0.1% something weird happends, my session remember doesn't the proper value.
I have this in the page that inits the class:
$imgScreen = new img4crop();
// And here some settings like $imgScreen->setExtraJs($extraJs);
In this class I have a functions to save the $this data to an array:
private function save(){
$_SESSION['cropper'][$this->uniqid] = serialize($this);
}
In init-page.php I open a fancybox, loading the settings back into the class:
$info = unserialize($_SESSION['cropper'][$_GET['id']]);
// Should now be the same as $imgScreen
The problem
Sometimes $_SESSION['cropper'][$_GET['id']] is set to NULL. I have no clue why. This only happens very rarely. The key is set.
When I debug, and appendecho $_SESSION['cropper'][$this->uniqid]; to the save() function, it gives me the value I want it to have.
So, where does my session value go, only in that rare occation? Why isn't it there in the fancybox screen?
After 1st comment: It is possible for the init-page to start multiple instances, they each get their own value in $_SESSION['cropper']. In the situation above, they all work, just that one doesnt. On reload, I start new instances and the exact same screen doesnt work, even though I now has a new, fresh id
Edit: It gets weirder: If the location where the image has a file with the same name as the class will use, there are no problems. Weird part: The whole class has no code regarding to files, apart from the final step (the screen has 3 steps), where I use ImageMagick.
Important to know: When I installed the page where this occurs, the targetfile didn't exist either, started blank.

Answer 1

When you are serializing objects you have to make sure you load the base class (prototype) before unserializing the object. For PHP native objects you don't have to do anything, it will load the base classes.
Your variable $this is an instance of a class. In order for $this to exist the base class need to exist. Perhaps it's working most of the time because of an autoloader or the particular sequence you are loading items.

Answer 2

Turns out the solution was, as to be expected, a quircky little something unrelated to the actual code:
In my index.php I do $_SESSION['cropper'] = array(); to reset the array, then let the function which shows an editorscreen set new settings to the cropper, this works perfectly fine.
In that html I have an image with an anti cache:
echo '<img src="'.$image_url.'?'.time.'" />';
The problem occured when $image_url is empty/undefined, resulting in a source something like the following: ?1420793438. It starts with a ?, so it's effectively calling index.php?1420793438.
And at the beginning of that index.php we reset, resulting in an empty array.