PHP register_globals on/off
  • The register_globals setting controls how you access form,server, and environment variables. 
  • By default this variable is set to Off.
  • When on, register_globals will inject your scripts with all sorts of variables.
  • when register_globals is set to ON in don’t need to write $_POST['var'] to access the posted ‘var’ variable you can simply use ‘$var’ to access the $_POST['var'].

It's preferred to use $GLOBALS,$_SERVER,$_GET,$_POST,$_FILES,$_COOKIE,$_SESSION,
$_REQUEST,$_ENV, rather than relying upon register_globals being on.

please check below link to view complete details:
php register_globals FAQ