Strange issues with header output

Hi there, I have been building a player script which outputs a windows media asx playlist with theheader output type of video-ms-wmv. I have experienced some wierd issues when i use ifstatements at certain points with the header tag enclosed within it, i get unexpected results on themac plugin. "Playlist format is unrecognised" Ie if ($_SESSION['referer_checked']) { header(aplicationtype ...); echo $asx_playtlist here } Is there somehow that an if statement is adding stuff to the header when it shouldnt be ? Therecould also be the possibility the plugin doesnt start a session or something, even though itsloading a php script as its filename ? Let me know what could be the issues.