Following T121597#4652873 and T121597#4681823 I talked with @joe and we agreed that we need to take a close look at our INI settings and make sure they are as identical as possible, with any differences either fixed or justified+documented.
With the very limited use of PHP 7 so far, we've already encountered one major difference which is that our current PHP 7 settings partially disable `error_reporting`, which is a problem. See also T187147 about that.
Todo from T211488#5102911:
* [x] Differences in APC settings.
* [x] Differences in Filesytem settings.
* [x] Differences in Serialisation settings.
* [x] Differences in Error handling.
* [x] Differences in File Uploads settings.
*  Differences in Session settings.
* [x] Differences in Mail settings.
* [x] Intl settings.
* [x] OPCache settings.
* [x] Redis settings.
* [ ] Mysql settings.
* [ ] Memcached settings.
* [x] Decide on PCRE JIT.
* [ ] Decide on `enable_dl`. See T220681
* [x] Fix `include_path`.
* [x] Fix `doc_root `.
* [x] curl.namedPools
* [x] Hard limit
* [x] Stat cache