fopen url with using post variable

hello

i'm using a php script opening en parsing a url like

www.OpenThisURL.com/site.php?a=aa&b=bb

this url and website are not mine but i'm only using some information
from the site.

now this site has changed and the GET variable a=aa & b=bb have
changed into POST variable. (i know because before i go to that page
i've to fill in a form and see that this form post method is POST)


the script i used was something like this:

$fp = fopen("www.OpenThisURL.com/site.php?a=aa&b=bb", "r");
$str = "";
while(! feof($fp))
$str .= fread($fp, 128);

fclose($fp);


how do i do something like that opening an url with some POST
variable??

thank you

.