oops, it should be
$out = <<<EOF
<a href="{$image_upload_iframe_src}&TB_iframe=true" id="add_image" class="thickbox" title='$image_title'><img src='images/media-button-image.gif' alt='$image_title' /></a>
<a href="{$video_upload_iframe_src}&TB_iframe=true" id="add_video" class="thickbox" title='$video_title'><img src='images/media-button-video.gif' alt='$video_title' /></a>
<a href="{$audio_upload_iframe_src}&TB_iframe=true" id="add_audio" class="thickbox" title='$audio_title'><img src='images/media-button-music.gif' alt='$audio_title' /></a>
<a href="{$media_upload_iframe_src}&TB_iframe=true" id="add_media" class="thickbox" title='$media_title'><img src='images/media-button-other.gif' alt='$media_title' /></a>
EOF;