[Colin F]

exactly, you need another field (like userid) to only update the one you changed (and not all of them).

$DB_site->query("UPDATE user SET stock = '$wholething' WHERE userid = $stock[userid]");

you'll also have to fetch the userid first, making it all together:

PHP Code:

$upstck = $DB_site->query("SELECT userid,stock FROM user"); 
while($stock = $DB_site->fetch_array($upstck)) 
{ 
$firstpart = substr($stock[stock],0,2); 
$lastpart = substr($stock[stock],4); 
$wholething = $firstpart . "00" . $lastpart; 
$DB_site->query("UPDATE user SET stock = '$wholething' WHERE userid = $stock[userid]");
}