oppure:

$query = "SELECT id FROM campo WHERE id>condizione";
$result = mysql_query($query, $db);
$numero = mysql_num_rows($result);

echo "Ho trovato $numero record";

$array=Array();
$max=0;
for($i=0;$i<$num;$i++)
{
$id=mysql_result($result,$i,"id");
$array[$i]=$id;
echo "$array[$i]
";
}

for($i=0;$i<$num;$i++)
{
if($array[$i]>$max)
$max=$array[$i];
}

echo $max;

--------------------------------------------

se devi solo trovare il max nn serve creare un array, basta fare:

$query = "SELECT id FROM campo WHERE id>condizione";
$result = mysql_query($query, $db);
$numero = mysql_num_rows($result);

echo "Ho trovato $numero record";

$max=0;
for($i=0;$i<$num;$i++)
{
$id=mysql_result($result,$i,"id");
if($id>$max)
$max=$id;
}

echo $max;