Php update database if checkbox is not checked

so I have a dynamically generated table

<tr>
    <td><?php echo $row['RequestID']?></td>
    <td><?php echo $row['FirstName'] . " " . $row['LastName']?></td>
    <td><?php echo $row['DateRequested']?></td>
    <td><?php echo $row['DateNeeded']?></td>
    <td><?php echo $row['TotalPrice']?></td>
    <td><?php echo $row['Company']?></td>
    <td><?php echo $row['Account']?></td>
    <td><?php echo $row['Brand']?></td>
    <td><?php echo $quantity?></td>
    <td><input type="checkbox" name="bill[]" value=<?php echo '"' . $row['RequestID'] . '"'; if($row['BillBack'] == "1") {echo "checked='checked'"; } ?></td>
</tr>

and I want to update the database when a row is checked or unchecked.

if(isset($_POST['submit'])){//to run PHP script on submit
    if(!empty($_POST['bill'])){
    // Loop to store and display values of individual checked checkbox.
    foreach($_POST['bill'] as $selected){
    echo "hello";
    $sql2 = "UPDATE Requests SET BillBack = '1' WHERE RequestID = '$selected'";
    $result2 = $conn->query($sql2);

    }} elseif(empty($_POST['bill'])){
    // Loop to store and display values of individual checked checkbox.
    foreach($_POST['bill'] as $selected){
    echo "hello";
    $sql2 = "UPDATE Requests SET BillBack = '0' WHERE RequestID = '$selected'";
    $result2 = $conn->query($sql2);
    }}

         }

Now. The if statement works like a charm. For every checked checkbox on a submit, that record gets updated. The issue comes with unchecking the checkboxes. As you can see, checked boxes are found by finding what is not empty. But, when I delete that exclamation point, it does not effectively do the opposite. It also doesn't work as it's own if statement, as a straight else statement, and checking for !isset. Is this because of how the inputs are deemed checked? is there a syntax error? I've done about 5 hours of error checking, googling, and the like... and I can't find shit. Thanks guys