Random unique ids

I'm trying to generate something like 6B6E23518 using randomString() which I'm calling inside my controller

 function randomString($chars=10) //generate random string
{
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $randstring = '';
    for ($i = 0; $i < $chars; $i++) {
        $randstring .= $characters[rand(0, strlen($characters))];
    }
    return $randstring;
}

public function store(TicketsCreateRequest $request)
{
    $ticket = $user->tickets()->create([

            'ticket_hash' => $this->randomString(10),
            // ....


    ]);
}

but this keeps on storing 0 into 'ticket_hash' and nothing gets generated??

via Chebli Mohamed