$n = "32";
function getRandomString($n)
{
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $randomString = '';

    for ($i = 0; $i < $n; $i++) {
        $index = rand(0, strlen($characters) - 1);
        $randomString .= $characters[$index];
    }

    return $randomString;
}

若要生成16位的随机字符串：

getRandomString(16);