Welcome to SE! The message is "Could not open input file" so the problem is finding the files it needs to run, not the username/password. What is 'my_php_script' in the cron entry? Does the cron entry literally include '[mysite]' or is that where you have redacted your site name?
There are several ways to run scheduled jobs but I suggest you use ...
Can you try reproducing on https://wpmaster.demo.civicrm.org/ (I did try and wasn't able to).
Looks like its a environment issue. Could you please check your htaccess or server config? Or WP permlinks?