I found this answer here:
if(($twitter_XML_raw=file_get_contents($timeline))==false){
// Retrieve HTTP status code
list($version,$status_code,$msg) = explode(' ',$http_response_header[0], 3);
// Check the HTTP Status code
switch($status_code) {
case 200:
$error_status="200: Success";
break;
case 401:
$error_status="401: Login failure. Try logging out and back in. Password are ONLY used when posting.";
break;
case 400:
$error_status="400: Invalid request. You may have exceeded your rate limit.";
break;
case 404:
$error_status="404: Not found. This shouldn't happen. Please let me know what happened using the feedback link above.";
break;
case 500:
$error_status="500: Twitter servers replied with an error. Hopefully they'll be OK soon!";
break;
case 502:
$error_status="502: Twitter servers may be down or being upgraded. Hopefully they'll be OK soon!";
break;
case 503:
$error_status="503: Twitter service unavailable. Hopefully they'll be OK soon!";
break;
default:
$error_status="Undocumented error: " . $status_code;
break;
}
Essentially, you use the "file get contents" method to retrieve the URL, which automatically populates the http response header variable with the status code.