Something

<?php

if(isset($_POST['text'])) {
  $f = fopen("info.txt", "w");
  fwrite($f, str_replace("<?", "NO PHP.", $_POST['text']));
  fclose($f);
  echo 'wroted it!';
  exit;
}

$f = fopen("info.txt", "r");
$t = fread($f, filesize("info.txt"));

echo '<form action="' . $_SERVER['PHP_SELF'] . '" method="post">';

echo '<textarea name="text" rows=43 cols=80>';
echo $t;
echo '</textarea><br><input type="submit"></form>';

?>