I ended up finding out that the mobile site we were trying to pass the cookie from was an HTML file, not PHP. We ended up creating a quick JavaScript snippet of code and it works perfect now.
Just insert this code into the header of the mobile page:
<script type="text/javascript">
var now = new Date();
var time = now.getTime();
time += 3600 * 1000;
now.setTime(time);
document.cookie="mobile=m; expires=" + now.toUTCString() + "; domain=.yourdomain.com; path=/";
</script>