вот дурачьё! =)

Code:
do {
    $rand = rand(124,987);
    $Db_user = substr($Db_name,0,4).$rand;

    // Test for unique new contact name.
    $sql = "SELECT * FROM clients WHERE user_id = '$Db_user'";

    $result = mysql_query($sql) or die("could not execute FIND MEMBER query");
} while (mysql_num_rows($result) > 0);
не благодари. . ^^,