We recently have had a significant number of people coming to us for Zen Cart upgrades needed due to web hosts changing the minimum PHP version they support. Unfortunately a lot them have been at the point that the website is broken due to that PHP change happening, so now would be good time to assess whether you are in need of an upgrade before you run in to problems.
If you upgrade to the latest version of Zen Cart, 1.5.6, you have years of support built in as that is designed for PHP versions up to PHP 7.3, which is supported until December of 2021.
While the lowest supported version of PHP is currently 7.1, one of the recent situations where we were brought in involved the web host raising the minimum PHP version to 5.6, which Zen Cart 1.5.3 and above were designed for.
Making the upgrade process more difficult in some instances is that one change made with Zen Cart 1.5.6 is that it will not run with versions of PHP lower than 5.5 (previous Zen Cart versions would run on older versions of PHP enough to test things out before switching to a newer version).