Chart is confusing, but would think one would know IF one is on:
shared hosting, VPS Hosting with cPanel, VPS Hosting with Plesk, or Linux Dedicated Servers, etc..
From their chart, PHP 5.4.x is default on Shared Hosing, but PHP 5.5x is by 'Configuration Plugin' ... whatever that means.
You say it works on another instance. Well, is that one under the same environment/setup? Shared, VPS, Linux Dedicated what? The one working on a different plan, different server(s), different domain, etc.?
Ok, still using Google here and get this (which indicates users could shoot themselves in the foot):
http://support.hostgator.com/articles/cpanel/php-configuration-plugin
This is really turning into assistance on HostGator rather than Moodle. Don't think the 'fault' is with Moodle but configuration of your HostGator setups. In many years Moodle usage/installs, don't think I've ever run across Moodle not finding PHP (version that was installed), but that's because I don't use shared hosting, VPS with either cPanel/Plesk, etc. but, rather, dedicated servers.
Hmmmm ... anyone out here in Moodleland host on HostGator that would/could help Peter?
'spirit of sharing', Ken