PHP Updating Data of grades in each subject

Question

I'm having trouble updating the data of grades in each subject I'm doing this and I can't figure the answer.

What is happening is when I press the submit button to update the grades of one subject all data of all subject will have the same value with the one I updated.

Here is my code:

<table>
    <thead>
        <tr>
            <th width="12%">Code</th>
            <th>Subject</th>
            <th width="10%">Units</th>
            <th width="10%">Grades</th>
            <th></th>
        </tr>
    </thead>

        <?php 
                echo "<form action='student_grade.php?studentID=".$std_id."' method='POST'>";

                while($student=mysql_fetch_array($records_3)){

                $subject = $student['subject_name'];
                $units = $student['subject_units'];
                $code = $student['subject_code'];
                $grade = $student['grade'];

                echo "<tr>";
                echo "<td>".$code."</td>";
                echo "<td>".$subject."</td>";
                echo "<td>".$units."</td>";
                echo "<td>".$grade."</td>";
                echo "<td><select name='grade'>
                        <option value='1'>      1       </option>
                        <option value='1.25'>   1.25    </option>
                        <option value='1.5'>    1.5     </option>
                        <option value='1.75'>   1.75    </option>
                        <option value='2'>      2       </option>
                        <option value='2.25'>   2.25    </option>
                        <option value='2.75'>   2.75    </option>
                        <option value='3'>      3       </option>
                        <option value='4'>      4       </option>
                        <option value='5'>      5       </option>
                        <option value='INC'>    INC     </option>
                        <option value='DRP'>    DRP     </option>
                        <option value='' selected>  -   </option>
                    </select>
                    </td>";
            }
        ?>
</table>

and here is my update query

if (isset($_POST['save'])){
    $UpdateQuery = "UPDATE student_subject SET grade='$_POST[grade]' WHERE student_id='$_POST[hidden]'";


    mysql_query($UpdateQuery,$con);
    echo "<meta http-equiv='refresh' content='0'>";
}

GRADES UI:

Answer 1

You can use simple variables in strings, like

$foo = "bar is $bar";

but to use more complicated variables ($array[index] or $object->value->value), you need to escape them, as

$foo = "bar[x] = {$bar['x']}";