Need to enable gd for an older version of PHP

The PHP version is 5.6, so quite old. Using apt-get install php5-gd won't work any more. I can locate these gd.so: /usr/lib/php/20151012/gd.so /usr/lib/x86_64-linux-gnu/libgd.so.3 -- links to 3.0.0 below /usr/lib/x86_64-linux-gnu/libgd.so.3.0.0 To enable gd, I think I have to set up one of these in mods_available, mods_enabled, etc. I'm guessing I'll need an alias gd.so in a common place which links to one of the above, but where to put it, and which one to link to? Is there a step-by-step guide on manually setting this up?