if(isset($_POST['btn']) and !empty($_POST['mytext']))
{
    $mytext = $_POST['mytext'];
    $file=fopen("123.txt", "r+");
    fwrite ($file, $mytext);
    fclose($file);
} 
  
  if(isset($_POST['btn']) and !empty($_POST['mytext']))
{
    $mytext = $_POST['mytext'];
    file_put_contents('123.txt', PHP_EOL . $mytext, FILE_APPEND);
}