Right now we are using Moodle 3.1 with PHP 5.6.
We are planning to upgrade Moodle 3.5.
Which version of PHP 7 should I upgrade.
Should I upgrade PHP before going to Moodle 3.2?
I read documentation and it says that Moodle 3.3 can be run with PHP 5.6.
You can upgrade straight from Moodle 3.1 to Moodle 3.5 but the only version of PHP compatible with both is 7.0. So, in that case I would upgrade to PHP 7.0, upgrade Moodle to 3.5 and then consider upgrading PHP to 7.1/7.2.
Failing that, you can upgrade to Moodle 3.3 which (as you say) is still compatible with PHP 5.6. Then upgrade to PHP 7.1. Then upgrade to Moodle 3.5.
Is it okay, if I upgrade PHP 7.0 for Moodle 3.1, and then once I upgrade moodle to 3.5, then I can upgrade PHP 7.1
Yep - isn't that what I said, though
