PHP：如何将用逗号分隔的年级字符串转换为英语可读的“年级范围”？

<?php
function toRange($string)
{
$min = \"K\";
$max = 5;

//take string and turn into an array
$grades = explode(\", \",$string);
$firstItem = $grades[0];
$lastItem = $grades[count($grades)-1];
if (count($grades) == 1)
{
  $output = $firstItem;
}
else
{
    if ($firstItem == \"K\")
    {
        if ($lastItem == 5)
        {
          $output = \"K & Up\";
        }
        if ($lastItem == 1)
        {
            $output = \"K & 1\";
        }
        else {
            $output = \"K -\" . $lastItem;
        }
        break;

    }
    else 
    {
        if ($lastItem == 5)
        {
            if ($firstItem != 4)
            {
                $output = $firstItem . \" & Up\";
            }
            else {
                $output = \"4 & 5\";
            }
        }
        else {
            if ($lastItem > $firstItem + 1)
            {
                $output = $firstItem . \" - \" . $lastItem;
            }
            else {
                $output = $firstItem . \" & \" . $lastItem;
            }
        }
    }


     }
return $output;
    }


?>