systemcheck.php should warn when attempting to install with an unsupported version of PHP

[gabeguz]

Web Store no longer supports PHP 5.3 and below. The systemcheck.php file should be updated to reflect this change.

[Goosse]

Minimum system requirement changes should also be at the top and in bold on release notes so that users do not update the store before they have the new minimum requirements installed on their system.

This just happened to me with this php change.