propagating session ID by URL

[sneakyimp]

i am currently augmenting my session handling code with a function that will add the current session ID to a url if necessary--like when users have their cookies turned off.

question 1: is it safe and reliable? to check the auto-defined constant SID to determine if i should append the session id or does definition of this constant depend on php ini settings or some other variables?

question 2: does this code look like it might work?

$pattern = '#' . SESSION_NAME . '=#';
if ((SID != '') && !preg_match($pattern, $url)) {
  if (strpos($url, '?') != false) {
    $url .= "&" . SID;
  } else {
    $url .= "?" . SID;
  }
}

i'm a bit concerned because if SID is not defined, then SID will not be equal to the empty string