I have googled without finding the answer but how do I make sure /all/ processes use php72 rather than the default 54 in CentOS 7? Surely there must be a better way than overwriting /usr/bin/php. What have I forgotten to do?
You can't/shouldn't do that. The point of the Enterprise OS is that versions are consistent throughout the lifetime of the OS, so packages don't change the default versions because things may break because something is expecting PHP 5.4 and not 7.2.
If you have an application that needs PHP 7.2, then change the way it is invoked so it runs using 'php72' instead of just 'php'.
Now, in all honesty I suspect that changing the default version of PHP isn't going to break many things at the system level; at least not in the same way as it would if you changed the default python to python3. But it is something that may save you problems in the future.
P.