<?php
if(isset($_POST['submit'])) {
$txt = $_POST['txt'];
$file = 'myfile.txt';
$fp = fopen($file, 'w');
fwrite($fp, $txt );
fclose($fp);
}
else {
print "<form action='' method='post'>
Текст: <input type='text' name='txt'><br>
<input type='submit' name='submit' value='Запиши'>";
}
?>