Comandare arduino in php

Guglio:
Prova

<?php $filename = "COM3"; $fp = fopen($filename, "r"); while (!feof($fp)) { echo fgets($fp, 128); } fclose($fp); ?>

mi da questo errore:
"Warning: fgets() expects parameter 1 to be resource, boolean given in C:\Program Files (x86)\EasyPHP-5.3.6.0\www\arduino_lettura.php on line 5

Warning: feof() expects parameter 1 to be resource, boolean given in C:\Program Files (x86)\EasyPHP-5.3.6.0\www\arduino_lettura.php on line 4"