Laravel's collection filter pass by reference not working

can you please help me in using closures. I'm using laravel's collection filter to check unique emails. The code below should fill the $unique_email array inside the filter function. Is there's a problem on how i used the pass by reference? Thanks in advance.

    $unique_email = array();

    $users->filter(function ($user) use (&$unique_email) {
        if( !in_array(   $user->email , $unique_email  ) ) {
            return $unique_email[] = $user->email;              
        }
    });

    dd($unique_email); //returns empty array

from Newest questions tagged laravel-5 - Stack Overflow http://ift.tt/2ePnDfI
via IFTTT