Randomly pick one out of table

[AnhTuanCool]

So hi people,

I have a small question and very needed to be explained
I want to randomly pick an id out of a table so I wrote this

$allids = $DB_site->query("SELECT id FROM " . TABLE_PREFIX . "test");

$idarray = array();
while ($ids = $DB_site->fetch_array($allids))
{
$idarray[] .= $ids['id'];
}

$id = array_rand($idarray);

But it didn't work, I know my PHP and SQL skill still bad so I really need some support now. Thanks in advanced.