$result = drupal_http_request(
  'http://example.com',
  array('Content-Type' => 'application/x-www-form-urlencoded'),
  'POST',
  'param1=value1&param2=value2'
);

$result = drupal_http_request('http://example.com', array(
  'headers' => array('Content-Type' => 'application/x-www-form-urlencoded'),
  'method' => 'POST',
  'data' => 'param1=value1&param2=value2',
));

Drupal 6, Drupal 7