Task is not deleted

<?php
require 'inc/functions.php';

$page = "tasks";
$pageTitle = "Task List | Time Tracker";

if (isset($_POST['delete'])) {
    if (delete_task(filter_input(INPUT_POST, 'delete', FILTER_SANITIZE_NUMBER_INT))) {
        header('location: task_list.php?msg=Task+Deleted');
        exit;
    } else {
        header('location: task_list.php?msg=Unable+to+Delete+Task');
        exit;
    }
}

if (isset($_GET['msg'])) {
    $error_message = trim(filter_input(INPUT_GET, 'msg', FILTER_SANITIZE_STRING));
}
include 'inc/header.php';
?>
<div class="section catalog random">

    <div class="col-container page-container">
        <div class="col col-70-md col-60-lg col-center">

            <h1 class="actions-header">Task List</h1>
            <div class="actions-item">
                <a class="actions-link" href="task.php">
                    <span class="actions-icon">
                        <svg viewbox="0 0 64 64"><use xlink:href="#task_icon"></use></svg>
                    </span>
                Add Task</a>
            </div>
            <?php
            if (isset($error_message)) {
                echo "<p class='message'>$error_message</p>";
            }
            ?>
            <div class="form-container">
              <ul class="items">
                <?php
                    foreach (get_task_list() as $item) {
                        echo "<li><a href='task.php?id=" . $item['task_id'] . "'>" . $item['title'] . "</a>";

                        echo "<form method='post' action='task_list.php'>\n";
                        echo "<input type='hidden' value='" . $item['task_id'] . "' name='delete'/>\n";
                        echo "<input type='submit' class='button--delete' value='Delete' />\n";
                        echo "</form>";
                        echo "</li>";
                    }
                ?>
              </ul>
            </div>

        </div>
    </div>
</div>

<?php include("inc/footer.php"); ?>

function delete_task($task_id) {
    include 'connection.php';
    $sql = 'DELETE task_id, title, date, time, project_id FROM tasks WHERE task_id = ?';    

    try {
        $results = $db->prepare($sql);
        $results->bindValue(1, $task_id, PDO::PARAM_INT);
        $results->execute();
    } catch (Exception $e) {
        echo"Error!: ". $e->getMessage() . "<br />";
        return false;
    }

    return true;    
}