<form method='post'>
<textarea name='tekst'></textarea><br/>
<input type='password' name='haslo' /><br/>

<input type='submit' value='Enter' /><br/><br/><br/>
</form>

<?php
$file=fopen('tresc.txt','r');
if(filesize('tresc.txt')>0) $tresc = fread($file, filesize('tresc.txt'));
else $tresc='';
fclose($file);

if( isset($_POST['tekst'])
 && isset($_POST['haslo'])
 && $_POST['haslo']=='qwertyuiop' )
{
  $tresc=$tresc.htmlspecialchars($_POST['tekst']).'<br/><br/>';

  $file=fopen('tresc.txt', 'w');
  fwrite($file, $tresc);
  fclose($file);
}

print $tresc;

?>