If you use Heart Internet and want to use php 5.6 or php version 7 on heart internet then you cannot change the PHP version above 5.5 in the Extend Control Panel. However you can change the PHP version to 5.6 or 7.0 using other means.
Both PHP 5.6 and 7.0 are available on the Heart Internet platform and can be accessed via editing the .htaccess file within the package to the following:
SetEnv DEFAULT_PHP_VERSION 56
SetEnv DEFAULT_PHP_VERSION 70
So, to recap, use the first line to change the PHP version to 5.6.3 and the second line to PHP version 7.0.14.
You can edit your .htaccess file by logging into your extend control panel. Visiting File Manager and then go to public_html and you can either add a new .htaccess or edit your existing one.