Scripts breaking when adding snippet

I have a snippet as below which works but when I use it the scripts on my site break (footer scripts, admin scripts, fly out cart etc.). I’ve tried different options in terms of auto adding the PHP tags and changing the snippet but all those cause critical errors. Please can you advise? The snippet content is as below:

<iframe class="trackingframe" src="https://global.cainiao.com/detail.htm?mailNoList=<?php echo $_SERVER['QUERY_STRING']; ?>" width="100%" frameborder="0" scrolling="no">

Thanks