<?
$mysqli = new mysqli('хост', 'юзер', 'пасс', 'база');
$flag = true;
do {
    $num = rand(1, 100);
    $q = $mysqli->fetch_assoc($mysqli->query("SELECT COUNT(uniq_number) AS pizda FROM huj WHERE uniq_number = '$num'"));
    if($q['pizda'] == 0)
        $flag = false;
}while($flag);
//тут юзай $num
?>