php version for subdomain with cagefs and php selector + ioncube loader 6

I went through the upgrade process to WHMCS 7 last week. The approach I recommend is to first upgrade to WHMCS 7 while you are running PHP 5.6. Once you have everything set up and working, including your crons, email piping, etc., then you can switch to PHP 7.

You don't mention it, but I guess you're using EA4? If so, once you're ready to switch to PHP 7, the first thing you need to do is make sure you have all the PHP extensions necessary for WHMCS 7 to run with PHP 7. Notably, iconv is required for WHMCS to run PHP 7, but not for PHP 5.6. Just double-check the system requirements for WHMCS 7 to make sure you have everything.

Once you've done that, log in to WHM and go Software -> MultiPHP Manager. Enter "dev3" in the search box to quickly find the subdomain you're working with. Check the box by the subdomain, then in the drop-down at the top-right select ea-php7 and click apply. If PHP 7 is not the system PHP version; i.e., you're still using PHP 5.6 for that, you're using a directory outside of the subdomain for your crons, and you want to use PHP 7 for crons, you'll need to copy the PHP handler section from the .htaccess file in the dev3 public_html folder to an .htaccess file in the crons folder.