This should be saved as 'thispage.php'
<?php
$text = $_POST['textadd'];
?>
<u>This is your text, so far</u>:<br><br>
<?php echo $text ?><br><br>
<FORM action='thispage.php' method='POST'>
<TEXTAREA NAME='textadd' COLS='40' ROWS='6'><?php echo $text ?></TEXTAREA>
<BR>
<INPUT TYPE='submit' VALUE='Submit'>
</FORM>
Hope I got it right 😃
This is a basic start,
to develope further.