PhP, MySql - Optimising Code

I am not a great php coder (I come from C++). I am using php only for database entry.

I have a database with the following:

UserId (an unique int)
AsyncPointsAverage (float)
AsyncPointsAverageRank (a position based on the value immediately above)
AsyncPointsRecentAverage (float an average for the last 5 tests only)
AsyncPointsRecentAverageRank (a position based on the value immediately above)

There are about 1000-1500 entries in that table. Every morning and afternoon 5 people take a test which effects their overall average and recent average. (This is updated elsewhere, but not shown here.) After that is calculated for those 5 people, then the rankings of all 1000-1500 will be effected, so I have written the code below. Is it optimal?

The thing I am most concerned with is I am doing a MySql UPDATE about a 1000 times. Is that great? Should I be doing it another way? (Also feel free to optimise any other code in the function. As I say, I am from a C++ background, so do not really know the nuances of php.)

// Sorts by array entry 1
function ReRankCompareAverage($a, $b)
{
    if($a[1] == $b[1]) return 0;
    else return ($a[1] > $b[1] ? 1 : -1);
}
// Sorts by array entry 2
function ReRankCompareAverageRecent($a, $b)
{
    if($a[2] == $b[2]) return 0;
    else return ($a[2] > $b[2] ? 1 : -1);
}

function ReRank($db)
{
    $i = 0, $j = 0;
    $usersARR = null;

    $stmt = $db->prepare("SELECT UserId, AsyncPointsAverage, AsyncPointsRecentAverage FROM studenttable");
    $stmt->execute();
    if($stmt && isset($stmt) && $stmt->rowCount() > 0)
    {
        $i = 0;
        while(($row = $stmt->fetch(PDO::FETCH_ASSOC)))
        {
            $usersARR[$i][0] = intval($row['UserId']);
            $usersARR[$i][1] = floatval($row['AsyncPointsAverage']);
            $usersARR[$i][2] = floatval($row['AsyncPointsRecentAverage']);
            $i++;
         }
    }
    $stmt->closeCursor(); // mysql_free_result equivalent

    // The first pass of $j == 3 does the ranking by Average, filling position $usersARR[][3] with that rank
    // The second pass of $j == 4 does the ranking by AverageRecent, filling position $usersARR[][4] with that rank
    for($j = 3, $j <= 4; $j++)
    {
        $iCompare = $j == 3 ? 1 : 2;

        usort($usersARR, $j == 3 ? "ReRankCompareAverage" : "ReRankCompareAverageLast");
        $count = count($usersARR);
        if($count > 0)
        {
            // Start it off, with the person with the highest average is rank 1
            $usersARR[$count - 1][$j] = 1; // Position $j is filled with the rank
            // Now loop starting from the second one down
            for($i = $count - 2, $rank = 1; $i >= 0; $i--)
            {
                // Only change the rank if the next one down is strictly lower than the one above, otherwise will share the same rank
                if($usersARR[$i][$iCompare] < $usersARR[$i+1][$iCompare]) $rank = $count - $i; // Otherwise keep the same rank, because they are equal
                $usersARR[$count - 1][$j] = $rank;
            }
        }
     }

     // Now $usersARR is filled with the correct rankings, and they are asscoiated with $UserId
    // Now we must put all of these rankings into the database
    $count = count($usersARR);
    for($i = 0; $i < $count; $i++)
    {
         $stmt = $db->prepare("UPDATE studenttable SET AsyncPointsAverageRank=:AsyncPointsAverageRank, AsyncPointsRecentAverageRank=:AsyncPointsRecentAverageRank "
                        . "WHERE UserId=:UserId");
         $stmt->execute(array(':AsyncPointsAverageRank' => $usersARR[$i][3],
                        ':AsyncPointsRecentAverageRank' => $usersARR[$i][4],
                        ':UserId' => $usersARR[$i][0]));
    }
}