To-do list using PHP and MySQLi

<?php
// Fill in your database hostname, username, password and database name below

$mysqli = new mysqli("localhost", "root", "", "to_do_list");
if(!$mysqli)
{
	die("Error: Cannot connect to the database");
}
?>

ADD TASK SCRIPT

<?php
// Check to be sure that the only request we can process is POST REQUEST
if(isset($_SERVER["REQUEST_METHOD"]) && strip_tags($_SERVER["REQUEST_METHOD"]) == strip_tags("POST"))
{
	if(isset($_POST['add']))
	{
		if( isset($_POST['task']) && !empty($_POST['task']) ) // Be sure the task brought here is not an empty field
		{
			// Variable declaration
			$task = trim(strip_tags(htmlspecialchars($_POST['task'])));
			
			// Save the task into the database
			mysqli_query($mysqli, "insert into `task` values(0, '".mysqli_real_escape_string($mysqli, $task)."', '".mysqli_real_escape_string($mysqli, 'Pending')."')") or die(mysqli_errno());
			
			mysqli_query($mysqli, "delete from `task` order by `task_id` asc limit 1");
			
			echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
		}
		else
		{
			echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
		}
	}
	else
	{
		echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
		//die('Sorry, no proper data was passed');
	}
}
else 
{
	echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
	// Deny access if the request brought to this page is not a POST REQUEST
	//die('Access Denied');
}
?>

UPDATE TASK SCRIPT

<?php
// Check to be sure that the only request we can process is POST REQUEST
if(isset($_SERVER["REQUEST_METHOD"]) && strip_tags($_SERVER["REQUEST_METHOD"]) == strip_tags("GET"))
{
	
	if(isset($_GET['task_id']) && !empty($_GET['task_id'])) // Be sure the task ID brought here is not an empty field
	{
		// Variable declaration
		$task_id = trim(strip_tags($_GET['task_id']));
		
		// Update the database to set the task to Completed
		mysqli_query($mysqli, "update `task` set `status` = '".mysqli_real_escape_string($mysqli, 'Completed')."' where `task_id` = '".mysqli_real_escape_string($mysqli, $task_id)."' limit 1") or die(mysqli_errno());
		
		echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
	}
	else
	{
		echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
		//die('Sorry, no proper data was passed');
	}
}
else 
{
	echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
	// Deny access if the request brought to this page is not a GET REQUEST
	//die('Access Denied');
}
?>

DELETE TASK SCRIPT

<?php
// Check to be sure that the only request we can process is POST REQUEST
if(isset($_SERVER["REQUEST_METHOD"]) && strip_tags($_SERVER["REQUEST_METHOD"]) == strip_tags("GET"))
{
	if(isset($_GET['task_id']) && !empty($_GET['task_id'])) // Be sure the task ID brought here is not an empty field
	{
		// Variable declaration
		$task_id = trim(strip_tags($_GET['task_id']));
		
		// Delete the task from the database
		mysqli_query($mysqli, "delete from `task` where `task_id` = '".mysqli_real_escape_string($mysqli, $task_id)."' limit 1") or die(mysqli_errno());
		
		echo "<script>window.location='index.php'</script>"; // Redirect back to index.php page
	}	
	else
	{
		echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
		//die('Sorry, no proper data was passed');
	}
}
else 
{
	echo "<script>window.location='index.php'</script>";  // Redirect back to index.php page
	// Deny access if the request brought to this page is not a GET REQUEST
	//die('Access Denied');
}
?>

The script is very easy to understand and customize with programming code comments to ease usability.