Mysql – Php, MySql – Optimisation

I asked this on the general site, but they suggested I come here for ultimate optimisation. It really boils down to which is faster; doing a load of calculations in php with database stuff that is accessed once, or doing two database calls. (There are about 1000-1500 table entries, and I have to use the lot.)

Here is my original question:

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]));
    }
}

After several answers, including not having any rankings in the database, but doing the rankings instead on-the-fly with:

SET @rank=0; SELECT @rank := @rank +1 AS rank, UserId, AsyncPointsAverage FROM studenttable ORDER BY AsyncPointsAverage DESC

This obviously has one draw-back, namely it does not cater for the follwoing scenario: if the 2nd and 3rd ranked people have 88%, they will both be ranked 2nd. The next person will then be ranked 4th (leaving rank 3rd out entirely.)

Another person has suggested I do something like:

SQL

SELECT
    UserId,
    AsyncPointsAverage,
    AsyncPointsAverageRank
FROM
    studenttable
ORDER BY
    AsyncPointsAverage DESC

PHP

$stmt = $db->prepare("SEE ABOVE...");
$stmt->execute();

if( $stmt && isset( $stmt ) && $stmt->rowCount() ) {
    $rank = 1;
    $last_grade = -1;

    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']);

        if( $usersARR[$i][1] < $last_grade ) {
            $rank++;
        }

        $usersARR[$i][3] = $rank;

        $last_grade = $usersARR[$i][1];
    }
}

but obviously this has to be done again for the RECENT Average, ie you are doing two calls to the database.

So please be super-critical of my code. I am not a php or mysql expert, so I will not be offended in any way. The way I see it, I want to learn.