Toggle sorting options with select and check boxes

<?php

include 'list.php';

$filter = array();
$status = '';
$checkThis = '';
$selectPrio = '';
$selectDue = '';
$selectTitle = '';
$field = '';

if(isset($_POST['field'])) {
    $field = $_POST['field'];
    if ($field == null) {
        echo 'Please select an option.';
    } else {
        switch ($field) {
            case 'priority':
                $selectPrio = 'selected';
                break;
            case 'due':
                $selectDue = 'selected';
                break;
            case 'title':
                $selectTitle = 'selected';
                break;
        }
    }
}

if(isset($_POST['status'])) {
    $status = 'all';
    $checkThis = 'checked';
}

echo '<form action="index.php" method="post">';
echo '<p>Sort by</p>';
echo '<select name="field">';
echo '<option value="priority"' . "$selectPrio" . '>Priority</option>';
echo '<option value="due"' . "$selectDue" . '>Due Date</option>';
echo '<option value="title"' . "$selectTitle" . '>Title</option>';
echo '</select>';
echo '<br>';
echo '<input type="checkbox" name="status" value="Yes"' . "$checkThis" . '>Show completed tasks';
echo '<br>';
echo '<br>';
echo '<input type="submit" value="Submit">';
echo '</form>';


foreach ($list as $originalKey => $item) {
    if ($status == 'all' || $item['complete'] == $status) {
        if (isset($field) && isset($item[$field])) {
            $filter[$originalKey] = $item[$field];
        } else {
            $filter[$originalKey] = $item['priority']+12;
        }
    }
}
asort($filter);

echo '<br>';
echo '<table>';
echo '<tr>';
echo '<th>Title</th>';
echo '<th>Priority</th>';
echo '<th>Due Date</th>';
echo '<th>Complete</th>';
echo '</tr>';
foreach($filter as $id => $value){
    echo '<tr>';
    echo '<td>' . $list[$id]['title'] . "</td>\n";
    echo '<td>' . $list[$id]['priority'] . "</td>\n";
    echo '<td>' . $list[$id]['due'] . "</td>\n";
    echo '<td>'; 
    if ($list[$id]['complete']){
        echo 'Yes';
    } else {
        echo 'No';
    }
    echo "</td>\n";
    echo '</tr>';
}
echo '</table>';

?>