The register_globals setting controls how you access form, 
server, and environment variables. By default this variable 
is set to Off, requiring you to use special arrays to 
access these variables.

As same answer. By setting it on we can access the
environment variables using it's variable name

When on, register_globals will inject your scripts with all 
sorts of variables, like request variables from HTML forms. 
This coupled with the fact that PHP doesn't require 
variable initialization means writing insecure code is that 
much easier. It was a difficult decision, but the PHP 
community decided to disable this directive by default. 
When on, people use variables yet really don't know for 
sure where they come from and can only assume. Internal 
variables that are defined in the script itself get mixed 
up with request data sent by users and disabling 
register_globals changes this.