There was somewhere the info at this forum that minimum PHP version number is 5.6 (But I don’t remember why). Maybe you will find this unswer from PKP team usefull: OJS 3.1: Why Exactly This Requires PHP 5.6? - #8 by asmecher
Interesting, and did you ask you technical support why they tend to use a quite outdated version of PHP? It was released 5 years ago and its support has ended in 2016.