taking down the system

I haven't played with setting variables in _SERVER global in this manner nor have I seen it in the wild.

You could go with a lock file solution to track when the cron starts and ends for your other files

Along the lines of

PHP Code:

file_put_contents('cron.lock', time());
//do your cron things here
unlink('cron.lock'); 


In your index, you can do

PHP Code:

if(file_exists('cron.lock'))
{
show_error("The Substantiator is down for weekly maintanance. ",500,"System down");
exit(0);
}