codice:
$text = get_file_contents("nomefile.txt");

// modifica $text

put_file_contents("nomefile.txt", $text);
dove se non hai PHP5

codice:
function get_file_contents($filename)
{
  if(file_exists($filename))
  {
    if(is_readable($filename))
    {
      if($hfile = fopen($filename, "r"))
      {
       $content = fread($hfile, filesize($filename));
       fclose($hfile);
       return($content);
      }
    }
  }
  return(false);
}

function put_file_contents($filename, $content)
{
  
    if($hfile = fopen($filename, "w"))
    {
       $res = fwrite($hfile, $content);
       fclose($hfile);
       return($res);
    }
 
  return(false);
}