Why my PHP array is null

It's all about scope in this case.

Because you have two $dirtyElements on that is global in your method and one that is in your closure.

$dirtyElements = array(); //The global one

    $collection = $items->each(function ($item, $key) use ($dirtyElements) {
        if($item->isDirty()) {
            
            $dirtyElements[] = $item->getDirty(); // The local one in the closure.
            //dd($dirtyElements); //The local one.
        }
    });

dd($dirtyElements) //The global one

I suggest you read this http://php.net/manual/en/functions.anonymous.php

You can pass $dirtyElements in by reference by prefixing with &

however, you would be better using collections to filter to just those items that are dirty and then call getdirty on those

something like

$dirtyElements = $items->filter(function ($item) {
        return $item->isDirty();
    })
    ->each($function($item) {
        return $item->getDirty();
    });

@Tray2 your answer and your reference were very helpful. Thanks.

@Snapey your example looked like nice and clean but I'm afraid each method does not work like that. I mean, I wanted dirty array but in that way it only returned me the whole model again. But your answer helped me to resolve my problem. Thanks.