Supported way to install additional php extensions

I have a client wishing to install Magento and all is OK except we are missing some required php extensions

mcrypt intl mbstring gd

We have php 5.6.5 running form the supported package how do we go about adding these missing extensions without breaking anything?

Many Thanks, Andrew



As an update I have php 5.6 installed and running per

So I went ahead and browsed this repository and found all the missing extensions and installed them except for mcrypt so I just need assistance getting mcrypt for 5.5 or maybe another option would be a how to add php7 using supported repository that contained mcrypt?