PHP writing to textbox

Hi there, I wish to write a set $var into a textbox.
 
So far, I have gotten:

<?php
    $result = "Hello world!"; //Do your logic anywhere but be sure to set the result here
?>
<form method="post" action="script.php">
....
<input type="text" value="<?= $result?>" readOnly>
<input type="submit">
</form>

Now, this sets my textbox to say "Hello World", however, I would like for the text box to be blank and once I have clicked my button, for the textbox to be populated with "hello world", not before.

Thank you for reading this.