write to file PHP

i need to write to file (not on server, on PC) after submitting form.

for example this one:
syndoc.com/media/inde.php

so if you fill the form, press a submit button, it will add text of textboxes into file: C:\text.txt

I tried this code, but it doesn't work :(
please help!!!


<form method="post" action="inde.php">
<input name="a1" type="text" value="a1" />
<input name="b2" type="text" value="b2" />
<input name="c3" type="text" value="c3" />
<input name="d4" type="submit" value="d4" />
</form>

<?php


if (isset($_post['d4']))
{
$string = "<div>\n";
$string .= "<div>".$_post['a1']."</div>\n";
$string .= "<div>".$_post['b2']."</div>\n";
$string .= "<div>".$_post['c3']."</div>\n";
$string .= "</div>";


$fp = fopen('c:\text.txt', 'w');

if ($fp)
{
fwrite($fp,$string);
fclose($fp);

echo 'Data written ok..';

}

}

?> inde