How to check is html element is deleted in php?

I have some page with same html elements:

<form action="" method="post" id="form" class="form">
    <div id="div3">
    <input type="hidden" name="3" id="3" hidden="true" value="27">
    <input type="text" name="task3" id="task3" class="task input" placeholder="Enter Task" required="required" value="SOME VALUE">
    <input class="input hover-to-show" type="button" name="removetask3" id="removetask3" value="Remove" onclick="remove(3)">
    </div> //'3' is an id of the element

    <input style="position: fixed; right: 0;" name="done" type="submit" value="Save" />
</form>

And there is a script remove(j):

function remove(j) {
    id--;
    let element = document.getElementById("div" + j);
    let input = document.getElementById("task"+j);
    input.setAttribute("value", " ");
    element.parentNode.removeChild(element);
}

And then i have php code with SQL statements:

if(isset($_POST["done"])){
    for($i = 1; $i < $count; $i++){
            if(htmlspecialchars(trim($_POST["task".$i])))
            {
                $index = $_POST[$i];
                $task = htmlspecialchars(trim($_POST["task".$i]));
                $imp = htmlspecialchars(trim($_POST["mark".$i]));
                mysqli_query($db, "UPDATE `data` SET `tasks`='$task',`imp`= '{$imp}' WHERE `id` = {$index} AND `customerid`={$id}");
                if($i > $rnum){
                    mysqli_query($db, "INSERT INTO `data` (`customerid`, `tasks`, `imp`) VALUES ({$id}, '{$task}', {$imp})");
                }
            }
            else{
                mysqli_query($db, "DELETE FROM `data` WHERE customerid={$id} AND id={$_POST[$i]}");
            }
        }
}

But

if(htmlspecialchars(trim($_POST["task".$i]))) {} is true for the deleted index

What is a possible solution to check, is element deleted or not?