Hi Kevin,
AtoM 2.4 currently supports PHP 7.0 when you follow our Ubuntu 16.04 instructions. Versions 7.1 and 7.2 introduce some non-backwards compatible changes that we've discovered cause issues in AtoM, so we need to do a bit more work to make AtoM function properly with those PHP versions.
Since Ubuntu 14.04 is also reaching EOL, for the 2.5 release we will be retiring the 14.04 documentation, and we'll be doing fixes and upgrades so we can release docs for installing AtoM with Ubuntu 18.04 and hopefully PHP 7.2.
Cheers,