The wiki currently states that CiviCRM has bare minimum (5.3.10) and recommended (>= 5.4) PHP requirements.
Knowing what CiviCRM has been tested with can help identify what should be expected to work reliably.
- As of today, what versions of PHP do CiviCRM's test infrastructure test with?
- This question will become out of date over time; for future reference, is there a better place to check this information?
- For orgs who aren't able to match the PHP versions that CiviCRM is tested with internally, what other options are available?