função que nomeia e move os arquivos

public function addFileToServer($imagem){

        $characters = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
        $result = '';
        for ($i = 0; $i < 10; $i++)
            $result .= $characters[mt_rand(0, 61)];

        $target_dir = $_SERVER['DOCUMENT_ROOT'] . "/ead/assets/images/duvidas/";
        $array = explode('.', $_FILES['imagem']['name']);
        $extension = end($array);
        $target_file = $target_dir . $result . "." . $extension;
        if (move_uploaded_file($_FILES["imagem"]["tmp_name"], $target_file)) {
            echo BASE . "assets/images/duvidas/" . $result . "." . $extension;
        }
    }