Get a list of names of all global variables in PHP

I'm working with Moodle and theres a lot of global variables and it seems that there's no proper documentation. First I thought like "yeah just see what `echo "<pre>",print_r($GLOBALS,1),"</pre>";` will yield". But when I try it, my browser becomes unresponsive while rendering and the rendered result is a mess. Unfortunately I don't have the possibility to use an debugger since the xdebug package is not available for the distro I'm using. (Turnkey Moodle appliance) So my question is: Is there a way to see just the names of all global variables so that I can inspect them individually?