<?php
$tmp = getdate();
$timedate = $tmp['mday'].".".
                   $tmp['mon'].".".
                   $tmp['year']."_".
                   $tmp['hours'].".".
                   $tmp['minutes'].".".
                   $tmp['seconds']."Uhr";


if(isset($_POST['save'])) {
  $inhalt  = $_POST['inhalt_textarea']."\n";

  $data = fopen("./".$timedate.".txt","a+");
  fwrite($data,$inhalt);
  fclose($data);
}
?>

<form method="POST">
<textarea name="inhalt_textarea" cols="30" rows="10"></textarea>
<input type="submit" name="save" value="speichern">
</form>

<form onchange="this.submit()" method="POST">
<textarea name="inhalt_textarea"  cols="30" rows="10"></textarea>

</form>