You can change your PHP version to whichever you prefer to use, from the list of PHP versions offered on Johnny.
If you install software packages from Softaculous, they'll sometimes require a specific PHP version, but they'll say this during the install. However if you're building your own PHP app, just choose the one you want from the available list.
Please note that you will not be able to run both Node and PHP on the same subdomain. If you want to build a Node app and also a PHP app, you'd need separate subdomains for them. You can have up to 10 total domains/subdomains on your account, so if you want a 2nd domain, please follow the steps here and create a new request thread, and we can make the changes for you.