А так ?
$source = $_POST['source'];
$file = "file.txt";
$Saved_File = fopen($file, 'a+');
fwrite($Saved_File, $source);
fclose($Saved_File); $source = $_POST['source'];
$file = "file.txt";
$Saved_File = fopen($file, 'a+');
fwrite($Saved_File, $source);
fclose($Saved_File);