Ranking and storing scores to a database
Formatting


your indentation is off, making your code hard to read.
it is customary to put the opening curly bracket on the same line as the statement it belongs to (except for functions and classes).
if (cond) return true; else return false; can be written as return cond.
Your spacing is not consistent (use an IDE to fix this - and most other formatting issues - easily).


If we fix these issues, the first function for example would look like this:

function gradeScoreFormula($score1, $score2)
{
    return $score1 &gt; $score2;
}


Naming


gradeScoreFormula is quite specific for such a generic function, yet it doesn't really tell me anything about what the function actually does. isLargerThan might be better. [Although the function seems all wrong anyways. All the sort functions expect the function to return an integer; in that case, you could simply call the function compare]


Security

You should use prepared statements, as you may otherwise be vulnerable to second order SQL injection.

Even if the score and/or uid values in the users table are not currently user input, they may be in the future. 

Additionally, prepared statements may increase performance in this case, as the query would only need to be prepared once.

Misc


WHERE uid = $k AND score = $v seems odd to me. Why are you checking for the score as well? If there is a reason, I would add a comment about it. If there is no reason, I would remove that check; the primary id should be enough, as far as I can tell.
why have position and a? It doesn't seem necessary to have two variables here. For that matter, it doesn't seem necessary to have either one, you could simply use i + 1 directly in the query.
Your foreach($reversedArr[$i] as $k =&gt; $v) is confusing and unnecessary. You only have it to access the key and value of a one-entry array. If you need the key, you can use the key function.


Complexity

Your code is indeed overly complex, mainly because you perform unnecessary and complex transformations between data structures and because you are not using the correct functions.

The core of your program could be as simple as this:

function gradeScore(array $values, $dbc)
{
    arsort($values);

    $position = 0;
    $query = "UPDATE users set subposition = ? WHERE uid = ?";
    $statement = $dbc-&gt;prepare($query);
    foreach($values as $id =&gt; $score) {
        $position++;
        $statement-&gt;bind_param("ss", $position, $id);
        $result = $statement-&gt;execute();
    }
}