form.php

<?php
ini_set("display_errors","1");

session_start();

if(isset($_POST['delete']))
{
    unset($_SESSION['task'][$_POST['delete']]);
}
else
{

    $task = $_POST['task'];
    if(!isset($_POST['task']))
    {
        $_SESSION['task'] = array();
    }

    if($task !== '')
    {
        $_SESSION['task'][] = $task;
    }
}
//
header('Location: tasks.php')
?>