No, that's not what I said. Many pick their own 'poison' for the OS - me, I tend to favor CentOS. Matthew is correct ... Googled possible solutions for you, that's all.
However, having said that, thought it interesting what that Google search found ... the issue over wording in the license being the reason php-json not included in Ubuntu builds of PHP. CentOS has it's issues also ... based upon RedHat Enterprise. IF RHEL doesn't include something for some reason, either CentOS folks have to make it available or folks have to use well known/reliable third party repo's that are know to supply those extensions etc. not provided by either (like webtatic).
Most moodle docs *might* be written for Ubuntu ... but, in the past, have seen CentOS also. Not sure that's a "recommendation by proxy", however, but rather, a lack of time/knowledge in providing directions. Not hard to translate, however - sudo apt-get = sudo yum or just yum and there are some differences in switches and the package names ... webtatic has php54w and php55w rather than just php.
Both OS's, BTW, maintain security fixes/patches and Ubuntu might have the edge there. CentOS, based upon RH, doesn't provide updates/fixes until RH does. RH is more 'conservative'. I just happen to think that if one errors one needs to error on the conservative side - would rather not be on the 'bleeding edge' ... but close to the 'leading edge'.
Sorry for expansion of this topic and the 'sidebar', but ...
Glad you have it fixed and you are up and running.
'spirit of sharing', Ken