php ajax debug method -- make log in a text file

//
ob_start();
echo "<pre>";
print_r($data);
echo "</pre>";
$a=ob_get_contents();
//DAL::remove("insert into mytest(vvv) values('$a')");
$filename = "file.txt";
$file = fopen($filename, "w");      //open file
fwrite($file, $a);                  //first line
fclose($file);                      //close
ob_end_clean();
//