用php遍历mysql表以查找字母成绩的平均成绩

我试图找到插入表中的一组成绩的GPA，成绩列为varchar类型，我需要循环遍历然后将其转换为十进制，我非常迷失，现在不知道如何进行此操作。任何帮助，将不胜感激

这是我到目前为止的东西，我不断收到错误消息：

    
        
    
    
Your Grades and GPA Calculator
    

    

    
\r\n";
       $result = mysqli_query($con, "SELECT * FROM gpa");
    while($row = mysqli_fetch_array($result)) {
        echo "            \r\n";
        echo "                \r\n";
        echo "                \r\n";
        echo "                \r\n";
        echo "                \r\n";
        echo "                \r\n";
        echo "            \r\n";
    }
    echo "        

        

            

            
Course Heading
            
Course Number
            
Credit Hours
            
Grade
        
" . $row['id'] . "
" . $row['heading'] . "
" . $row['courseNUM'] . "
" . $row['creditHours'] . "
" . $row['grade'] . "
\r\n"; 
    echo "
Your overall GPA:" .$totalGrade;
}
mysqli_close($con);

?>