Clarification on Send me a Copy checkbox

Hey everyone,

I know the bit of checkbox for “Send me a copy” has been asked and answered.

As of 2014 onwards, somebody shared his solution here (around halfway down the page:
https://www.motomax.de/wordpress/contact-form-7-send-copy-to-sender-with-checkbox-in-3-9-update/

But the thing I was wondering when I tested this solution, I had to change the [0] of:
if($posted_data[‘contact_sendmail’][0]) {

to [‘contact_sendmail’][1] after following the rest of the instructions (except for leaving the “To:” field blank in Mail(2),) to have it work. (checked for send, blank for not)

I wanted to ask why it worked for me that way?
I probably should be glad that it did, but if I had to change part of what was already presented in a solution, then I’m just concerned I might’ve changed something else from the plugin. But that I didn’t touch the plugin files, only created and filled out the necessary details of a form in the dashboard.

Hope someone can clarify this bit. I’m a also a noob with php and I’d appreciate if I can learn more.

Thank you!