You should put it this way:
- the minimum version XC 4.7.8 will work with is 5.3.x
- the minimum required is 5.6.x at the moment
- recommended - 7.x
Any PHP version below 5.6.x should not be used and after December 2018 5.6.x cannot be used either. -
http://php.net/supported-versions.php