Could someone explain to me what "Subscriber List Segments" is?
Just for posterity - when using this Campaign.Create if you want to pass through only a specific segment of a list then you can do it thusly
$subscriber_segments = array(array("ListID"=>$listid, "Name"=>"SEGMENT NAME"));
passing that through to the function as setup in the PHP wrapper
$result = $cm->campaignCreate( $cmapiClient, $campaign_name, $subject, $from_name, $from_email, $reply_email, $html_content, $text_content, $subscriber_listid, $subscriber_segments );