Apart from environmental stuff, the moodle_minimum_php_version_is_met()
function is executed by all installs (web, cli, database, ...)
and halts any installation (early) not meeting the PHP constraints.
* Introduce a new php compatible library which can be used
early in execution, without Moodle dependencies
* Use it where we previously had hardcoded checks
This was originally suggested in MDL-39007.