Add system check for required PHP extensions (bcmath, curl, etc.)
I was recently bitten by the bcmath requirement (along the lines of https://civicrm.stackexchange.com/q/31932/907), on a system that (for unrelated reasons) was making error messages fairly hard to get to.
It occurs to me that since CiviCRM core requires several php extensions (https://docs.civicrm.org/sysadmin/en/latest/requirements/#php-extensions), it would be helpful to have a system check that uses extension_loaded()
to confirm these are installed, and presents a 'critical' error if any are missing.