Hello im about to create a forge server and there is a checkbox if i want to intall php 5.6 instead of php 7. My question is, it is possible to encounter error or bug if I install php 7 on the server while on my local is still php 5.5.12 ?
Well yes, it's definitely a risk. PHP 7 has removed some old features. It's possible that your code still uses some of these. There are also a few other changes that might break things.
Upgrade your local or testing environment to PHP 7 first, and test your code, before you consider deploying PHP 7 in production.