<?php
require_once ( 'username.php' );
if (isset($_POST['textfield'])) {
echo username();
return;
}
echo '
<form name="form1" method="post" action="">
<p>
<label>
<input type="text" name="textfield" id="textfield">
</label>
</p>
<p>
<label>
<input type="submit" name="button" id="button" value="Submit">
</label>
</p>
</form>';
?>
You need to run the function in the page the form is sent to.