If you need to periodically sort by grades (A, A+, D-, etc.), here is a compare function that compares strings by the case-insensitive method, unless it finds a grade, in which case it correctly sorts by putting "plus" grades first, unmarked grades second, and "minus" grades last.

<?php
function cmp($a, $b)
{
   $a = preg_replace('@^(a|an|the) @', '', $a);
   $b = preg_replace('@^(a|an|the) @', '', $b);

   //special code for grades
   if (strpos( $a, "+") !== false || strpos( $b, "+") !== false  || 
       strpos( $a, "-") !== false || strpos( $b, "-") !== false ){

        $substrA = substr($a, 0, 1);
        $substrB = substr($b, 0, 1);        

        $modifierA = (strlen($a) == 2) ? substr($a, 1, 1) : "";
        $modifierB = (strlen($b) == 2) ? substr($b, 1, 1) : "";
    
        if ($substrA == $substrB){
            //figure out plusses and minuses.
            if ($modifierA == "+"){
                return -1;
            } else if ($modifierB == "+"){
                return 1;
            }
        
            if ($modifierA == "-"){
                return 1;
            } else if ($modifierB == '-'){
                return -1;
            }
        } else {
            return strcasecmp($a, $b);
        }
 
    }
   return strcasecmp($a, $b);
}

$grades = array( 
                 "C+" => 13    ,
                "C" => 10     ,
                "D+" => 8     ,
                "B+" => 7     ,
                "C-" => 6     ,
                "A-" => 5     ,
                "F" => 5      ,
                "B" => 4      ,
                "B-" => 4     ,
                "D" => 3      ,
                "D-" => 3     ,
                "A+" => 1
    ); 

uksort($grades, "cmp");
?>

result:  Array
(
    [A+] => 1
    [A-] => 5
    [B+] => 7
    [B] => 4
    [B-] => 4
    [C+] => 13
    [C] => 10
    [C-] => 6
    [D+] => 8
    [D] => 3
    [D-] => 3
    [F] => 5
)