How to create a running list using new inputs (php)

How to create a running list using new inputs (php)

Basically I have a simple form and instead of refreshing the page each
time and replacing the echo just to add an echo for each time the form is
submitted. For reference I'm trying to create a to-do list, but this code
just refreshes the page and replaces the current echo.
<form action="#" method="post">
<input type="input" name="task">
<input type="submit" value="Add Task">
</form>
<?php
echo $_POST["task"];
?>