Thank you and that's really nice to say 
5.3.3 should still be working on PHP 5.6. It may be that it needs to be 5.6.5 or have certain dependencies switched on.
That said, it looks like Pulse 5.3.4 onwards (or so) will be PHP 7 or higher only. The survey is speaking for itself:
https://forum.pulsecms.com/t/can-we-drop-php-5-6/1551/3
And yes that will mean updating the installer checker also - good idea.
Thanks again 